Patchwork [v2,1/4] lib: introduce copy_struct_from_user() helper

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Submitter Aleksa Sarai
Date Sept. 25, 2019, 11:03 p.m.
Message ID <20190925230332.18690-2-cyphar@cyphar.com>
Download mbox | patch
Permalink /patch/34666/
State New
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Comments

Aleksa Sarai - Sept. 25, 2019, 11:03 p.m.
A common pattern for syscall extensions is increasing the size of a
struct passed from userspace, such that the zero-value of the new fields
result in the old kernel behaviour (allowing for a mix of userspace and
kernel vintages to operate on one another in most cases).

While this interface exists for communication in both directions, only
one interface is straightforward to have reasonable semantics for
(userspace passing a struct to the kernel). For kernel returns to
userspace, what the correct semantics are (whether there should be an
error if userspace is unaware of a new extension) is very
syscall-dependent and thus probably cannot be unified between syscalls
(a good example of this problem is [1]).

Previously there was no common lib/ function that implemented
the necessary extension-checking semantics (and different syscalls
implemented them slightly differently or incompletely[2]). Future
patches replace common uses of this pattern to make use of
copy_struct_from_user().

Some in-kernel selftests that insure that the handling of alignment and
various byte patterns are all handled identically to memchr_inv() usage.

[1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
     robustify sched_read_attr() ABI logic and code")

[2]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
     similar checks to copy_struct_from_user() while rt_sigprocmask(2)
     always rejects differently-sized struct arguments.

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 include/linux/bitops.h  |   7 +++
 include/linux/uaccess.h |   4 ++
 lib/strnlen_user.c      |   8 +--
 lib/test_user_copy.c    |  59 ++++++++++++++++++---
 lib/usercopy.c          | 115 ++++++++++++++++++++++++++++++++++++++++
 5 files changed, 180 insertions(+), 13 deletions(-)
Aleksa Sarai - Sept. 25, 2019, 11:07 p.m.
(Damn, I forgot to add Kees to Cc.)

On 2019-09-26, Aleksa Sarai <cyphar@cyphar.com> wrote:
> A common pattern for syscall extensions is increasing the size of a
> struct passed from userspace, such that the zero-value of the new fields
> result in the old kernel behaviour (allowing for a mix of userspace and
> kernel vintages to operate on one another in most cases).
> 
> While this interface exists for communication in both directions, only
> one interface is straightforward to have reasonable semantics for
> (userspace passing a struct to the kernel). For kernel returns to
> userspace, what the correct semantics are (whether there should be an
> error if userspace is unaware of a new extension) is very
> syscall-dependent and thus probably cannot be unified between syscalls
> (a good example of this problem is [1]).
> 
> Previously there was no common lib/ function that implemented
> the necessary extension-checking semantics (and different syscalls
> implemented them slightly differently or incompletely[2]). Future
> patches replace common uses of this pattern to make use of
> copy_struct_from_user().
> 
> Some in-kernel selftests that insure that the handling of alignment and
> various byte patterns are all handled identically to memchr_inv() usage.
> 
> [1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
>      robustify sched_read_attr() ABI logic and code")
> 
> [2]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
>      similar checks to copy_struct_from_user() while rt_sigprocmask(2)
>      always rejects differently-sized struct arguments.
> 
> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
>  include/linux/bitops.h  |   7 +++
>  include/linux/uaccess.h |   4 ++
>  lib/strnlen_user.c      |   8 +--
>  lib/test_user_copy.c    |  59 ++++++++++++++++++---
>  lib/usercopy.c          | 115 ++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 180 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index cf074bce3eb3..a23f4c054768 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -4,6 +4,13 @@
>  #include <asm/types.h>
>  #include <linux/bits.h>
>  
> +/* Set bits in the first 'n' bytes when loaded from memory */
> +#ifdef __LITTLE_ENDIAN
> +#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
> +#else
> +#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
> +#endif
> +
>  #define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE)
>  #define BITS_TO_LONGS(nr)	DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
>  
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 34a038563d97..824569e309e4 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -230,6 +230,10 @@ static inline unsigned long __copy_from_user_inatomic_nocache(void *to,
>  
>  #endif		/* ARCH_HAS_NOCACHE_UACCESS */
>  
> +extern int is_zeroed_user(const void __user *from, size_t count);
> +extern int copy_struct_from_user(void *dst, size_t ksize,
> +				 const void __user *src, size_t usize);
> +
>  /*
>   * probe_kernel_read(): safely attempt to read from a location
>   * @dst: pointer to the buffer that shall take the data
> diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
> index 7f2db3fe311f..39d588aaa8cd 100644
> --- a/lib/strnlen_user.c
> +++ b/lib/strnlen_user.c
> @@ -2,16 +2,10 @@
>  #include <linux/kernel.h>
>  #include <linux/export.h>
>  #include <linux/uaccess.h>
> +#include <linux/bitops.h>
>  
>  #include <asm/word-at-a-time.h>
>  
> -/* Set bits in the first 'n' bytes when loaded from memory */
> -#ifdef __LITTLE_ENDIAN
> -#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
> -#else
> -#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
> -#endif
> -
>  /*
>   * Do a strnlen, return length of string *with* final '\0'.
>   * 'count' is the user-supplied count, while 'max' is the
> diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> index 67bcd5dfd847..f7cde3845ccc 100644
> --- a/lib/test_user_copy.c
> +++ b/lib/test_user_copy.c
> @@ -31,14 +31,58 @@
>  # define TEST_U64
>  #endif
>  
> -#define test(condition, msg)		\
> -({					\
> -	int cond = (condition);		\
> -	if (cond)			\
> -		pr_warn("%s\n", msg);	\
> -	cond;				\
> +#define test(condition, msg, ...)					\
> +({									\
> +	int cond = (condition);						\
> +	if (cond)							\
> +		pr_warn("[%d] " msg "\n", __LINE__, ##__VA_ARGS__);	\
> +	cond;								\
>  })
>  
> +static int test_is_zeroed_user(char *kmem, char __user *umem, size_t size)
> +{
> +	int ret = 0;
> +	size_t start, end, i;
> +	size_t zero_start = size / 4;
> +	size_t zero_end = size - zero_start;
> +
> +	/*
> +	 * We conduct a series of is_zeroed_user() tests on a block of memory
> +	 * with the following byte-pattern (trying every possible [start,end]
> +	 * pair):
> +	 *
> +	 *   [ 00 ff 00 ff ... 00 00 00 00 ... ff 00 ff 00 ]
> +	 *
> +	 * And we verify that is_zeroed_user() acts identically to memchr_inv().
> +	 */
> +
> +	for (i = 0; i < zero_start; i += 2)
> +		kmem[i] = 0x00;
> +	for (i = 1; i < zero_start; i += 2)
> +		kmem[i] = 0xff;
> +
> +	for (i = zero_end; i < size; i += 2)
> +		kmem[i] = 0xff;
> +	for (i = zero_end + 1; i < size; i += 2)
> +		kmem[i] = 0x00;
> +
> +	ret |= test(copy_to_user(umem, kmem, size),
> +		    "legitimate copy_to_user failed");
> +
> +	for (start = 0; start <= size; start++) {
> +		for (end = start; end <= size; end++) {
> +			int retval = is_zeroed_user(umem + start, end - start);
> +			int expected = memchr_inv(kmem + start, 0, end - start) == NULL;
> +
> +			ret |= test(retval != expected,
> +				    "is_zeroed_user(=%d) != memchr_inv(=%d) mismatch (start=%lu, end=%lu)",
> +				    retval, expected, start, end);
> +		}
> +	}
> +
> +	return ret;
> +}
> +
>  static int __init test_user_copy_init(void)
>  {
>  	int ret = 0;
> @@ -106,6 +150,9 @@ static int __init test_user_copy_init(void)
>  #endif
>  #undef test_legit
>  
> +	/* Test usage of is_zeroed_user(). */
> +	ret |= test_is_zeroed_user(kmem, usermem, PAGE_SIZE);
> +
>  	/*
>  	 * Invalid usage: none of these copies should succeed.
>  	 */
> diff --git a/lib/usercopy.c b/lib/usercopy.c
> index c2bfbcaeb3dc..f795cf0946ad 100644
> --- a/lib/usercopy.c
> +++ b/lib/usercopy.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include <linux/uaccess.h>
> +#include <linux/bitops.h>
>  
>  /* out-of-line parts */
>  
> @@ -31,3 +32,117 @@ unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
>  }
>  EXPORT_SYMBOL(_copy_to_user);
>  #endif
> +
> +/**
> + * is_zeroed_user: check if a userspace buffer is full of zeros
> + * @from:  Source address, in userspace.
> + * @size: Size of buffer.
> + *
> + * This is effectively shorthand for "memchr_inv(from, 0, size) == NULL" for
> + * userspace addresses. If there are non-zero bytes present then false is
> + * returned, otherwise true is returned.
> + *
> + * Returns:
> + *  * -EFAULT: access to userspace failed.
> + */
> +int is_zeroed_user(const void __user *from, size_t size)
> +{
> +	unsigned long val;
> +	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
> +
> +	if (unlikely(!size))
> +		return true;
> +
> +	from -= align;
> +	size += align;
> +
> +	if (!user_access_begin(from, size))
> +		return -EFAULT;
> +
> +	unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> +	if (align)
> +		val &= ~aligned_byte_mask(align);
> +
> +	while (size > sizeof(unsigned long)) {
> +		if (unlikely(val))
> +			goto done;
> +
> +		from += sizeof(unsigned long);
> +		size -= sizeof(unsigned long);
> +
> +		unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> +	}
> +
> +	if (size < sizeof(unsigned long))
> +		val &= aligned_byte_mask(size);
> +
> +done:
> +	user_access_end();
> +	return (val == 0);
> +err_fault:
> +	user_access_end();
> +	return -EFAULT;
> +}
> +EXPORT_SYMBOL(is_zeroed_user);
> +
> +/**
> + * copy_struct_from_user: copy a struct from userspace
> + * @dst:   Destination address, in kernel space. This buffer must be @ksize
> + *         bytes long.
> + * @ksize: Size of @dst struct.
> + * @src:   Source address, in userspace.
> + * @usize: (Alleged) size of @src struct.
> + *
> + * Copies a struct from userspace to kernel space, in a way that guarantees
> + * backwards-compatibility for struct syscall arguments (as long as future
> + * struct extensions are made such that all new fields are *appended* to the
> + * old struct, and zeroed-out new fields have the same meaning as the old
> + * struct).
> + *
> + * @ksize is just sizeof(*dst), and @usize should've been passed by userspace.
> + * The recommended usage is something like the following:
> + *
> + *   SYSCALL_DEFINE2(foobar, const struct foo __user *, uarg, size_t, usize)
> + *   {
> + *      int err;
> + *      struct foo karg = {};
> + *
> + *      err = copy_struct_from_user(&karg, sizeof(karg), uarg, size);
> + *      if (err)
> + *        return err;
> + *
> + *      // ...
> + *   }
> + *
> + * There are three cases to consider:
> + *  * If @usize == @ksize, then it's copied verbatim.
> + *  * If @usize < @ksize, then the userspace has passed an old struct to a
> + *    newer kernel. The rest of the trailing bytes in @dst (@ksize - @usize)
> + *    are to be zero-filled.
> + *  * If @usize > @ksize, then the userspace has passed a new struct to an
> + *    older kernel. The trailing bytes unknown to the kernel (@usize - @ksize)
> + *    are checked to ensure they are zeroed, otherwise -E2BIG is returned.
> + *
> + * Returns (in all cases, some data may have been copied):
> + *  * -E2BIG:  (@usize > @ksize) and there are non-zero trailing bytes in @src.
> + *  * -EFAULT: access to userspace failed.
> + */
> +int copy_struct_from_user(void *dst, size_t ksize,
> +			  const void __user *src, size_t usize)
> +{
> +	size_t size = min(ksize, usize);
> +	size_t rest = max(ksize, usize) - size;
> +
> +	/* Deal with trailing bytes. */
> +	if (usize < ksize) {
> +		memset(dst + size, 0, rest);
> +	} else if (usize > ksize) {
> +		int ret = is_zeroed_user(src + size, rest);
> +		if (ret <= 0)
> +			return ret ?: -E2BIG;
> +	}
> +	/* Copy the interoperable parts of the struct. */
> +	if (copy_from_user(dst, src, size))
> +		return -EFAULT;
> +	return 0;
> +}
> -- 
> 2.23.0
>
Christian Brauner - Sept. 25, 2019, 11:21 p.m.
On Thu, Sep 26, 2019 at 01:03:29AM +0200, Aleksa Sarai wrote:
> A common pattern for syscall extensions is increasing the size of a
> struct passed from userspace, such that the zero-value of the new fields
> result in the old kernel behaviour (allowing for a mix of userspace and
> kernel vintages to operate on one another in most cases).
> 
> While this interface exists for communication in both directions, only
> one interface is straightforward to have reasonable semantics for
> (userspace passing a struct to the kernel). For kernel returns to
> userspace, what the correct semantics are (whether there should be an
> error if userspace is unaware of a new extension) is very
> syscall-dependent and thus probably cannot be unified between syscalls
> (a good example of this problem is [1]).
> 
> Previously there was no common lib/ function that implemented
> the necessary extension-checking semantics (and different syscalls
> implemented them slightly differently or incompletely[2]). Future
> patches replace common uses of this pattern to make use of
> copy_struct_from_user().
> 
> Some in-kernel selftests that insure that the handling of alignment and
> various byte patterns are all handled identically to memchr_inv() usage.
> 
> [1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
>      robustify sched_read_attr() ABI logic and code")
> 
> [2]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
>      similar checks to copy_struct_from_user() while rt_sigprocmask(2)
>      always rejects differently-sized struct arguments.
> 
> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
>  include/linux/bitops.h  |   7 +++
>  include/linux/uaccess.h |   4 ++
>  lib/strnlen_user.c      |   8 +--
>  lib/test_user_copy.c    |  59 ++++++++++++++++++---
>  lib/usercopy.c          | 115 ++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 180 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index cf074bce3eb3..a23f4c054768 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -4,6 +4,13 @@
>  #include <asm/types.h>
>  #include <linux/bits.h>
>  
> +/* Set bits in the first 'n' bytes when loaded from memory */
> +#ifdef __LITTLE_ENDIAN
> +#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
> +#else
> +#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
> +#endif
> +
>  #define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE)
>  #define BITS_TO_LONGS(nr)	DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
>  
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 34a038563d97..824569e309e4 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -230,6 +230,10 @@ static inline unsigned long __copy_from_user_inatomic_nocache(void *to,
>  
>  #endif		/* ARCH_HAS_NOCACHE_UACCESS */
>  
> +extern int is_zeroed_user(const void __user *from, size_t count);
> +extern int copy_struct_from_user(void *dst, size_t ksize,
> +				 const void __user *src, size_t usize);
> +
>  /*
>   * probe_kernel_read(): safely attempt to read from a location
>   * @dst: pointer to the buffer that shall take the data
> diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
> index 7f2db3fe311f..39d588aaa8cd 100644
> --- a/lib/strnlen_user.c
> +++ b/lib/strnlen_user.c
> @@ -2,16 +2,10 @@
>  #include <linux/kernel.h>
>  #include <linux/export.h>
>  #include <linux/uaccess.h>
> +#include <linux/bitops.h>
>  
>  #include <asm/word-at-a-time.h>
>  
> -/* Set bits in the first 'n' bytes when loaded from memory */
> -#ifdef __LITTLE_ENDIAN
> -#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
> -#else
> -#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
> -#endif
> -
>  /*
>   * Do a strnlen, return length of string *with* final '\0'.
>   * 'count' is the user-supplied count, while 'max' is the
> diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> index 67bcd5dfd847..f7cde3845ccc 100644
> --- a/lib/test_user_copy.c
> +++ b/lib/test_user_copy.c
> @@ -31,14 +31,58 @@
>  # define TEST_U64
>  #endif
>  
> -#define test(condition, msg)		\
> -({					\
> -	int cond = (condition);		\
> -	if (cond)			\
> -		pr_warn("%s\n", msg);	\
> -	cond;				\
> +#define test(condition, msg, ...)					\
> +({									\
> +	int cond = (condition);						\
> +	if (cond)							\
> +		pr_warn("[%d] " msg "\n", __LINE__, ##__VA_ARGS__);	\
> +	cond;								\
>  })
>  
> +static int test_is_zeroed_user(char *kmem, char __user *umem, size_t size)
> +{
> +	int ret = 0;
> +	size_t start, end, i;
> +	size_t zero_start = size / 4;
> +	size_t zero_end = size - zero_start;
> +
> +	/*
> +	 * We conduct a series of is_zeroed_user() tests on a block of memory
> +	 * with the following byte-pattern (trying every possible [start,end]
> +	 * pair):
> +	 *
> +	 *   [ 00 ff 00 ff ... 00 00 00 00 ... ff 00 ff 00 ]
> +	 *
> +	 * And we verify that is_zeroed_user() acts identically to memchr_inv().
> +	 */
> +
> +	for (i = 0; i < zero_start; i += 2)
> +		kmem[i] = 0x00;
> +	for (i = 1; i < zero_start; i += 2)
> +		kmem[i] = 0xff;
> +
> +	for (i = zero_end; i < size; i += 2)
> +		kmem[i] = 0xff;
> +	for (i = zero_end + 1; i < size; i += 2)
> +		kmem[i] = 0x00;
> +
> +	ret |= test(copy_to_user(umem, kmem, size),
> +		    "legitimate copy_to_user failed");
> +
> +	for (start = 0; start <= size; start++) {
> +		for (end = start; end <= size; end++) {
> +			int retval = is_zeroed_user(umem + start, end - start);
> +			int expected = memchr_inv(kmem + start, 0, end - start) == NULL;
> +
> +			ret |= test(retval != expected,
> +				    "is_zeroed_user(=%d) != memchr_inv(=%d) mismatch (start=%lu, end=%lu)",
> +				    retval, expected, start, end);
> +		}
> +	}
> +
> +	return ret;
> +}
> +
>  static int __init test_user_copy_init(void)
>  {
>  	int ret = 0;
> @@ -106,6 +150,9 @@ static int __init test_user_copy_init(void)
>  #endif
>  #undef test_legit
>  
> +	/* Test usage of is_zeroed_user(). */
> +	ret |= test_is_zeroed_user(kmem, usermem, PAGE_SIZE);
> +
>  	/*
>  	 * Invalid usage: none of these copies should succeed.
>  	 */
> diff --git a/lib/usercopy.c b/lib/usercopy.c
> index c2bfbcaeb3dc..f795cf0946ad 100644
> --- a/lib/usercopy.c
> +++ b/lib/usercopy.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include <linux/uaccess.h>
> +#include <linux/bitops.h>
>  
>  /* out-of-line parts */
>  
> @@ -31,3 +32,117 @@ unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
>  }
>  EXPORT_SYMBOL(_copy_to_user);
>  #endif
> +
> +/**
> + * is_zeroed_user: check if a userspace buffer is full of zeros
> + * @from:  Source address, in userspace.
> + * @size: Size of buffer.
> + *
> + * This is effectively shorthand for "memchr_inv(from, 0, size) == NULL" for
> + * userspace addresses. If there are non-zero bytes present then false is
> + * returned, otherwise true is returned.
> + *
> + * Returns:
> + *  * -EFAULT: access to userspace failed.
> + */
> +int is_zeroed_user(const void __user *from, size_t size)
> +{
> +	unsigned long val;
> +	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
> +
> +	if (unlikely(!size))
> +		return true;

You're returning "true" and another implicit boolean with (val == 0)
down below but -EFAULT in other places. But that function is
int is_zeroed_user()
Would probably be good if you either switch to
bool is_zeroed_user()
as the name suggests or rename the function and have it return an int
everywhere.

> +
> +	from -= align;
> +	size += align;
> +
> +	if (!user_access_begin(from, size))
> +		return -EFAULT;
> +
> +	unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> +	if (align)
> +		val &= ~aligned_byte_mask(align);
> +
> +	while (size > sizeof(unsigned long)) {
> +		if (unlikely(val))
> +			goto done;
> +
> +		from += sizeof(unsigned long);
> +		size -= sizeof(unsigned long);
> +
> +		unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> +	}
> +
> +	if (size < sizeof(unsigned long))
> +		val &= aligned_byte_mask(size);
> +
> +done:
> +	user_access_end();
> +	return (val == 0);
> +err_fault:
> +	user_access_end();
> +	return -EFAULT;
> +}
> +EXPORT_SYMBOL(is_zeroed_user);
> +
> +/**
> + * copy_struct_from_user: copy a struct from userspace
> + * @dst:   Destination address, in kernel space. This buffer must be @ksize
> + *         bytes long.
> + * @ksize: Size of @dst struct.
> + * @src:   Source address, in userspace.
> + * @usize: (Alleged) size of @src struct.
> + *
> + * Copies a struct from userspace to kernel space, in a way that guarantees
> + * backwards-compatibility for struct syscall arguments (as long as future
> + * struct extensions are made such that all new fields are *appended* to the
> + * old struct, and zeroed-out new fields have the same meaning as the old
> + * struct).
> + *
> + * @ksize is just sizeof(*dst), and @usize should've been passed by userspace.
> + * The recommended usage is something like the following:
> + *
> + *   SYSCALL_DEFINE2(foobar, const struct foo __user *, uarg, size_t, usize)
> + *   {
> + *      int err;
> + *      struct foo karg = {};
> + *
> + *      err = copy_struct_from_user(&karg, sizeof(karg), uarg, size);
> + *      if (err)
> + *        return err;
> + *
> + *      // ...
> + *   }
> + *
> + * There are three cases to consider:
> + *  * If @usize == @ksize, then it's copied verbatim.
> + *  * If @usize < @ksize, then the userspace has passed an old struct to a
> + *    newer kernel. The rest of the trailing bytes in @dst (@ksize - @usize)
> + *    are to be zero-filled.
> + *  * If @usize > @ksize, then the userspace has passed a new struct to an
> + *    older kernel. The trailing bytes unknown to the kernel (@usize - @ksize)
> + *    are checked to ensure they are zeroed, otherwise -E2BIG is returned.
> + *
> + * Returns (in all cases, some data may have been copied):
> + *  * -E2BIG:  (@usize > @ksize) and there are non-zero trailing bytes in @src.
> + *  * -EFAULT: access to userspace failed.
> + */
> +int copy_struct_from_user(void *dst, size_t ksize,
> +			  const void __user *src, size_t usize)
> +{
> +	size_t size = min(ksize, usize);
> +	size_t rest = max(ksize, usize) - size;
> +
> +	/* Deal with trailing bytes. */
> +	if (usize < ksize) {
> +		memset(dst + size, 0, rest);
> +	} else if (usize > ksize) {
> +		int ret = is_zeroed_user(src + size, rest);
> +		if (ret <= 0)
> +			return ret ?: -E2BIG;
> +	}
> +	/* Copy the interoperable parts of the struct. */
> +	if (copy_from_user(dst, src, size))
> +		return -EFAULT;
> +	return 0;
> +}
> -- 
> 2.23.0
>
Christian Brauner - Sept. 26, 2019, 12:40 p.m.
On Thu, Sep 26, 2019 at 01:03:29AM +0200, Aleksa Sarai wrote:
> A common pattern for syscall extensions is increasing the size of a
> struct passed from userspace, such that the zero-value of the new fields
> result in the old kernel behaviour (allowing for a mix of userspace and
> kernel vintages to operate on one another in most cases).
> 
> While this interface exists for communication in both directions, only
> one interface is straightforward to have reasonable semantics for
> (userspace passing a struct to the kernel). For kernel returns to
> userspace, what the correct semantics are (whether there should be an
> error if userspace is unaware of a new extension) is very
> syscall-dependent and thus probably cannot be unified between syscalls
> (a good example of this problem is [1]).
> 
> Previously there was no common lib/ function that implemented
> the necessary extension-checking semantics (and different syscalls
> implemented them slightly differently or incompletely[2]). Future
> patches replace common uses of this pattern to make use of
> copy_struct_from_user().
> 
> Some in-kernel selftests that insure that the handling of alignment and
> various byte patterns are all handled identically to memchr_inv() usage.
> 
> [1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
>      robustify sched_read_attr() ABI logic and code")
> 
> [2]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
>      similar checks to copy_struct_from_user() while rt_sigprocmask(2)
>      always rejects differently-sized struct arguments.
> 
> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
>  include/linux/bitops.h  |   7 +++
>  include/linux/uaccess.h |   4 ++
>  lib/strnlen_user.c      |   8 +--
>  lib/test_user_copy.c    |  59 ++++++++++++++++++---
>  lib/usercopy.c          | 115 ++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 180 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index cf074bce3eb3..a23f4c054768 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -4,6 +4,13 @@
>  #include <asm/types.h>
>  #include <linux/bits.h>
>  
> +/* Set bits in the first 'n' bytes when loaded from memory */
> +#ifdef __LITTLE_ENDIAN
> +#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
> +#else
> +#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
> +#endif

Nti: The style in bitops.h suggestes this should be:

+/* Set bits in the first 'n' bytes when loaded from memory */
+#ifdef __LITTLE_ENDIAN
+#  define aligned_byte_mask(n) ((1UL << 8*(n))-1)
+#else
+#  define aligned_byte_mask(n) (~0xffUL << (BITS_PER_LONG - 8 - 8*(n)))
+#endif

Using UL also makes 0xffUL clearer.

> +
>  #define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE)
>  #define BITS_TO_LONGS(nr)	DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
>  
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 34a038563d97..824569e309e4 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -230,6 +230,10 @@ static inline unsigned long __copy_from_user_inatomic_nocache(void *to,
>  
>  #endif		/* ARCH_HAS_NOCACHE_UACCESS */
>  
> +extern int is_zeroed_user(const void __user *from, size_t count);
> +extern int copy_struct_from_user(void *dst, size_t ksize,
> +				 const void __user *src, size_t usize);
> +
>  /*
>   * probe_kernel_read(): safely attempt to read from a location
>   * @dst: pointer to the buffer that shall take the data
> diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
> index 7f2db3fe311f..39d588aaa8cd 100644
> --- a/lib/strnlen_user.c
> +++ b/lib/strnlen_user.c
> @@ -2,16 +2,10 @@
>  #include <linux/kernel.h>
>  #include <linux/export.h>
>  #include <linux/uaccess.h>
> +#include <linux/bitops.h>
>  
>  #include <asm/word-at-a-time.h>
>  
> -/* Set bits in the first 'n' bytes when loaded from memory */
> -#ifdef __LITTLE_ENDIAN
> -#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
> -#else
> -#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
> -#endif
> -
>  /*
>   * Do a strnlen, return length of string *with* final '\0'.
>   * 'count' is the user-supplied count, while 'max' is the
> diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> index 67bcd5dfd847..f7cde3845ccc 100644
> --- a/lib/test_user_copy.c
> +++ b/lib/test_user_copy.c
> @@ -31,14 +31,58 @@
>  # define TEST_U64
>  #endif
>  
> -#define test(condition, msg)		\
> -({					\
> -	int cond = (condition);		\
> -	if (cond)			\
> -		pr_warn("%s\n", msg);	\
> -	cond;				\
> +#define test(condition, msg, ...)					\
> +({									\
> +	int cond = (condition);						\
> +	if (cond)							\
> +		pr_warn("[%d] " msg "\n", __LINE__, ##__VA_ARGS__);	\
> +	cond;								\
>  })
>  
> +static int test_is_zeroed_user(char *kmem, char __user *umem, size_t size)
> +{
> +	int ret = 0;
> +	size_t start, end, i;
> +	size_t zero_start = size / 4;
> +	size_t zero_end = size - zero_start;
> +
> +	/*
> +	 * We conduct a series of is_zeroed_user() tests on a block of memory
> +	 * with the following byte-pattern (trying every possible [start,end]
> +	 * pair):
> +	 *
> +	 *   [ 00 ff 00 ff ... 00 00 00 00 ... ff 00 ff 00 ]
> +	 *
> +	 * And we verify that is_zeroed_user() acts identically to memchr_inv().
> +	 */
> +
> +	for (i = 0; i < zero_start; i += 2)
> +		kmem[i] = 0x00;
> +	for (i = 1; i < zero_start; i += 2)
> +		kmem[i] = 0xff;
> +
> +	for (i = zero_end; i < size; i += 2)
> +		kmem[i] = 0xff;
> +	for (i = zero_end + 1; i < size; i += 2)
> +		kmem[i] = 0x00;
> +
> +	ret |= test(copy_to_user(umem, kmem, size),
> +		    "legitimate copy_to_user failed");
> +
> +	for (start = 0; start <= size; start++) {
> +		for (end = start; end <= size; end++) {
> +			int retval = is_zeroed_user(umem + start, end - start);
> +			int expected = memchr_inv(kmem + start, 0, end - start) == NULL;
> +
> +			ret |= test(retval != expected,
> +				    "is_zeroed_user(=%d) != memchr_inv(=%d) mismatch (start=%lu, end=%lu)",
> +				    retval, expected, start, end);
> +		}
> +	}
> +
> +	return ret;
> +}
> +
>  static int __init test_user_copy_init(void)
>  {
>  	int ret = 0;
> @@ -106,6 +150,9 @@ static int __init test_user_copy_init(void)
>  #endif
>  #undef test_legit
>  
> +	/* Test usage of is_zeroed_user(). */
> +	ret |= test_is_zeroed_user(kmem, usermem, PAGE_SIZE);
> +
>  	/*
>  	 * Invalid usage: none of these copies should succeed.
>  	 */
> diff --git a/lib/usercopy.c b/lib/usercopy.c
> index c2bfbcaeb3dc..f795cf0946ad 100644
> --- a/lib/usercopy.c
> +++ b/lib/usercopy.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include <linux/uaccess.h>
> +#include <linux/bitops.h>
>  
>  /* out-of-line parts */
>  
> @@ -31,3 +32,117 @@ unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
>  }
>  EXPORT_SYMBOL(_copy_to_user);
>  #endif
> +
> +/**
> + * is_zeroed_user: check if a userspace buffer is full of zeros
> + * @from:  Source address, in userspace.
> + * @size: Size of buffer.
> + *
> + * This is effectively shorthand for "memchr_inv(from, 0, size) == NULL" for
> + * userspace addresses. If there are non-zero bytes present then false is
> + * returned, otherwise true is returned.
> + *
> + * Returns:
> + *  * -EFAULT: access to userspace failed.
> + */
> +int is_zeroed_user(const void __user *from, size_t size)

See my bool vs int comment from yesterday and [1] for a suggestion.

> +{
> +	unsigned long val;
> +	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
> +
> +	if (unlikely(!size))
> +		return true;
> +
> +	from -= align;
> +	size += align;
> +
> +	if (!user_access_begin(from, size))
> +		return -EFAULT;
> +
> +	unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> +	if (align)
> +		val &= ~aligned_byte_mask(align);
> +
> +	while (size > sizeof(unsigned long)) {
> +		if (unlikely(val))
> +			goto done;
> +
> +		from += sizeof(unsigned long);
> +		size -= sizeof(unsigned long);
> +
> +		unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> +	}
> +
> +	if (size < sizeof(unsigned long))
> +		val &= aligned_byte_mask(size);
> +
> +done:
> +	user_access_end();
> +	return (val == 0);
> +err_fault:
> +	user_access_end();
> +	return -EFAULT;
> +}
> +EXPORT_SYMBOL(is_zeroed_user);


> +
> +/**
> + * copy_struct_from_user: copy a struct from userspace
> + * @dst:   Destination address, in kernel space. This buffer must be @ksize
> + *         bytes long.
> + * @ksize: Size of @dst struct.
> + * @src:   Source address, in userspace.
> + * @usize: (Alleged) size of @src struct.
> + *
> + * Copies a struct from userspace to kernel space, in a way that guarantees
> + * backwards-compatibility for struct syscall arguments (as long as future
> + * struct extensions are made such that all new fields are *appended* to the
> + * old struct, and zeroed-out new fields have the same meaning as the old
> + * struct).
> + *
> + * @ksize is just sizeof(*dst), and @usize should've been passed by userspace.
> + * The recommended usage is something like the following:
> + *
> + *   SYSCALL_DEFINE2(foobar, const struct foo __user *, uarg, size_t, usize)
> + *   {
> + *      int err;
> + *      struct foo karg = {};
> + *
> + *      err = copy_struct_from_user(&karg, sizeof(karg), uarg, size);
> + *      if (err)
> + *        return err;
> + *
> + *      // ...
> + *   }
> + *
> + * There are three cases to consider:
> + *  * If @usize == @ksize, then it's copied verbatim.
> + *  * If @usize < @ksize, then the userspace has passed an old struct to a
> + *    newer kernel. The rest of the trailing bytes in @dst (@ksize - @usize)
> + *    are to be zero-filled.
> + *  * If @usize > @ksize, then the userspace has passed a new struct to an
> + *    older kernel. The trailing bytes unknown to the kernel (@usize - @ksize)
> + *    are checked to ensure they are zeroed, otherwise -E2BIG is returned.
> + *
> + * Returns (in all cases, some data may have been copied):
> + *  * -E2BIG:  (@usize > @ksize) and there are non-zero trailing bytes in @src.
> + *  * -EFAULT: access to userspace failed.
> + */
> +int copy_struct_from_user(void *dst, size_t ksize,
> +			  const void __user *src, size_t usize)
> +{
> +	size_t size = min(ksize, usize);
> +	size_t rest = max(ksize, usize) - size;
> +
> +	/* Deal with trailing bytes. */
> +	if (usize < ksize) {
> +		memset(dst + size, 0, rest);
> +	} else if (usize > ksize) {
> +		int ret = is_zeroed_user(src + size, rest);
> +		if (ret <= 0)
> +			return ret ?: -E2BIG;
> +	}
> +	/* Copy the interoperable parts of the struct. */
> +	if (copy_from_user(dst, src, size))
> +		return -EFAULT;
> +	return 0;
> +}
> -- 
> 2.23.0
> 

[1]: How about:

/**
 * <sensible documentation>
 * 
 * Returns 1, if the user buffer is zeroed, 0 if it is not, and a
 * negative error code otherwise.
 * 
 */
int memuser_zero(const void __user *from, size_t size)
{
	unsigned long val;
	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);

	if (unlikely(size == 0))
		return 1;

	from -= align;
	size += align;

	if (!user_access_begin(from, size))
		return -EFAULT;

	unsafe_get_user(val, (unsigned long __user *) from, err_fault);
	if (align)
		val &= ~aligned_byte_mask(align);

	while (size > sizeof(unsigned long)) {
		if (unlikely(val))
			goto err_fault;

		from += sizeof(unsigned long);
		size -= sizeof(unsigned long);

		unsafe_get_user(val, (unsigned long __user *) from, err_fault);
	}

	if (size < sizeof(unsigned long))
		val &= aligned_byte_mask(size);

done:
	user_access_end();
	return (val == 0);
err_fault:
	user_access_end();
	return -EFAULT;
}

int copy_struct_from_user(void *dst, size_t ksize,
			  const void __user *src, size_t usize)
{
	size_t size = min(ksize, usize);
	size_t rest = max(ksize, usize) - size;

	/* Deal with trailing bytes. */
	if (usize < ksize) {
		memset(dst + size, 0, rest);
	} else if ((usize > ksize) {
 		int ret = memuser_zero(src + size, rest);
		if (ret < 0) /* we failed to check the user memory somehow */
			return ret;
		if (ret == 0) /* some of the memory was non-zero */
			return -E2BIG;
	}

	/* Copy the interoperable parts of the struct. */
	if (copy_from_user(dst, src, size))
		return -EFAULT;
	return 0;
}
Aleksa Sarai - Sept. 27, 2019, 1:07 a.m.
On 2019-09-26, Christian Brauner <christian.brauner@ubuntu.com> wrote:
> On Thu, Sep 26, 2019 at 01:03:29AM +0200, Aleksa Sarai wrote:
> > +int is_zeroed_user(const void __user *from, size_t size)
> > +{
> > +	unsigned long val;
> > +	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
> > +
> > +	if (unlikely(!size))
> > +		return true;
> 
> You're returning "true" and another implicit boolean with (val == 0)
> down below but -EFAULT in other places. But that function is int
> is_zeroed_user() Would probably be good if you either switch to bool
> is_zeroed_user() as the name suggests or rename the function and have
> it return an int everywhere.

I just checked, and in C11 (and presumably in older specs) it is
guaranteed that "true" and "false" from <stdbool.h> have the values 1
and 0 (respectively) [§7.18]. So this is perfectly well-defined.

Personally, I think it's more readable to have:

  if (unlikely(size == 0))
    return true;
  /* ... */
  return (val == 0);

compared to:

  if (unlikely(size == 0))
    return 1;
  /* ... */
  return val ? 0 : 1;

But I will change the function name (to check_zeroed_user) to make it
clearer that it isn't returning a boolean and that you need to check for
negative returns.
Christian Brauner - Sept. 27, 2019, 8:20 a.m.
On Fri, Sep 27, 2019 at 11:07:36AM +1000, Aleksa Sarai wrote:
> On 2019-09-26, Christian Brauner <christian.brauner@ubuntu.com> wrote:
> > On Thu, Sep 26, 2019 at 01:03:29AM +0200, Aleksa Sarai wrote:
> > > +int is_zeroed_user(const void __user *from, size_t size)
> > > +{
> > > +	unsigned long val;
> > > +	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
> > > +
> > > +	if (unlikely(!size))
> > > +		return true;
> > 
> > You're returning "true" and another implicit boolean with (val == 0)
> > down below but -EFAULT in other places. But that function is int
> > is_zeroed_user() Would probably be good if you either switch to bool
> > is_zeroed_user() as the name suggests or rename the function and have
> > it return an int everywhere.
> 
> I just checked, and in C11 (and presumably in older specs) it is
> guaranteed that "true" and "false" from <stdbool.h> have the values 1
> and 0 (respectively) [§7.18]. So this is perfectly well-defined.
> 
If you declare a function as returning an int, return ints and don't mix
returning ints and "proper" C boolean types. This:

static int foo()
{
	if (bla)
		return true;
	return -1;
}

is just messy.

> 
> Personally, I think it's more readable to have:
> 
>   if (unlikely(size == 0))
>     return true;
>   /* ... */
>   return (val == 0);
> 
> compared to:
> 
>   if (unlikely(size == 0))
>     return 1;
>   /* ... */
>   return val ? 0 : 1;

Just do:

if (unlikely(size == 0))
	return 1;
/* ... */
return (val == 0);

You don't need to change the last return.

Also, as I said in a previous mail: Please wait for rc1 (that's just two
days) to be out so you can base your patchset on that as there are
changes in mainline that cause a merge conflict with your changes.

Thanks!
Christian

Patch

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index cf074bce3eb3..a23f4c054768 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -4,6 +4,13 @@ 
 #include <asm/types.h>
 #include <linux/bits.h>
 
+/* Set bits in the first 'n' bytes when loaded from memory */
+#ifdef __LITTLE_ENDIAN
+#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
+#else
+#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
+#endif
+
 #define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE)
 #define BITS_TO_LONGS(nr)	DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
 
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 34a038563d97..824569e309e4 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -230,6 +230,10 @@  static inline unsigned long __copy_from_user_inatomic_nocache(void *to,
 
 #endif		/* ARCH_HAS_NOCACHE_UACCESS */
 
+extern int is_zeroed_user(const void __user *from, size_t count);
+extern int copy_struct_from_user(void *dst, size_t ksize,
+				 const void __user *src, size_t usize);
+
 /*
  * probe_kernel_read(): safely attempt to read from a location
  * @dst: pointer to the buffer that shall take the data
diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
index 7f2db3fe311f..39d588aaa8cd 100644
--- a/lib/strnlen_user.c
+++ b/lib/strnlen_user.c
@@ -2,16 +2,10 @@ 
 #include <linux/kernel.h>
 #include <linux/export.h>
 #include <linux/uaccess.h>
+#include <linux/bitops.h>
 
 #include <asm/word-at-a-time.h>
 
-/* Set bits in the first 'n' bytes when loaded from memory */
-#ifdef __LITTLE_ENDIAN
-#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
-#else
-#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
-#endif
-
 /*
  * Do a strnlen, return length of string *with* final '\0'.
  * 'count' is the user-supplied count, while 'max' is the
diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
index 67bcd5dfd847..f7cde3845ccc 100644
--- a/lib/test_user_copy.c
+++ b/lib/test_user_copy.c
@@ -31,14 +31,58 @@ 
 # define TEST_U64
 #endif
 
-#define test(condition, msg)		\
-({					\
-	int cond = (condition);		\
-	if (cond)			\
-		pr_warn("%s\n", msg);	\
-	cond;				\
+#define test(condition, msg, ...)					\
+({									\
+	int cond = (condition);						\
+	if (cond)							\
+		pr_warn("[%d] " msg "\n", __LINE__, ##__VA_ARGS__);	\
+	cond;								\
 })
 
+static int test_is_zeroed_user(char *kmem, char __user *umem, size_t size)
+{
+	int ret = 0;
+	size_t start, end, i;
+	size_t zero_start = size / 4;
+	size_t zero_end = size - zero_start;
+
+	/*
+	 * We conduct a series of is_zeroed_user() tests on a block of memory
+	 * with the following byte-pattern (trying every possible [start,end]
+	 * pair):
+	 *
+	 *   [ 00 ff 00 ff ... 00 00 00 00 ... ff 00 ff 00 ]
+	 *
+	 * And we verify that is_zeroed_user() acts identically to memchr_inv().
+	 */
+
+	for (i = 0; i < zero_start; i += 2)
+		kmem[i] = 0x00;
+	for (i = 1; i < zero_start; i += 2)
+		kmem[i] = 0xff;
+
+	for (i = zero_end; i < size; i += 2)
+		kmem[i] = 0xff;
+	for (i = zero_end + 1; i < size; i += 2)
+		kmem[i] = 0x00;
+
+	ret |= test(copy_to_user(umem, kmem, size),
+		    "legitimate copy_to_user failed");
+
+	for (start = 0; start <= size; start++) {
+		for (end = start; end <= size; end++) {
+			int retval = is_zeroed_user(umem + start, end - start);
+			int expected = memchr_inv(kmem + start, 0, end - start) == NULL;
+
+			ret |= test(retval != expected,
+				    "is_zeroed_user(=%d) != memchr_inv(=%d) mismatch (start=%lu, end=%lu)",
+				    retval, expected, start, end);
+		}
+	}
+
+	return ret;
+}
+
 static int __init test_user_copy_init(void)
 {
 	int ret = 0;
@@ -106,6 +150,9 @@  static int __init test_user_copy_init(void)
 #endif
 #undef test_legit
 
+	/* Test usage of is_zeroed_user(). */
+	ret |= test_is_zeroed_user(kmem, usermem, PAGE_SIZE);
+
 	/*
 	 * Invalid usage: none of these copies should succeed.
 	 */
diff --git a/lib/usercopy.c b/lib/usercopy.c
index c2bfbcaeb3dc..f795cf0946ad 100644
--- a/lib/usercopy.c
+++ b/lib/usercopy.c
@@ -1,5 +1,6 @@ 
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/uaccess.h>
+#include <linux/bitops.h>
 
 /* out-of-line parts */
 
@@ -31,3 +32,117 @@  unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
 }
 EXPORT_SYMBOL(_copy_to_user);
 #endif
+
+/**
+ * is_zeroed_user: check if a userspace buffer is full of zeros
+ * @from:  Source address, in userspace.
+ * @size: Size of buffer.
+ *
+ * This is effectively shorthand for "memchr_inv(from, 0, size) == NULL" for
+ * userspace addresses. If there are non-zero bytes present then false is
+ * returned, otherwise true is returned.
+ *
+ * Returns:
+ *  * -EFAULT: access to userspace failed.
+ */
+int is_zeroed_user(const void __user *from, size_t size)
+{
+	unsigned long val;
+	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
+
+	if (unlikely(!size))
+		return true;
+
+	from -= align;
+	size += align;
+
+	if (!user_access_begin(from, size))
+		return -EFAULT;
+
+	unsafe_get_user(val, (unsigned long __user *) from, err_fault);
+	if (align)
+		val &= ~aligned_byte_mask(align);
+
+	while (size > sizeof(unsigned long)) {
+		if (unlikely(val))
+			goto done;
+
+		from += sizeof(unsigned long);
+		size -= sizeof(unsigned long);
+
+		unsafe_get_user(val, (unsigned long __user *) from, err_fault);
+	}
+
+	if (size < sizeof(unsigned long))
+		val &= aligned_byte_mask(size);
+
+done:
+	user_access_end();
+	return (val == 0);
+err_fault:
+	user_access_end();
+	return -EFAULT;
+}
+EXPORT_SYMBOL(is_zeroed_user);
+
+/**
+ * copy_struct_from_user: copy a struct from userspace
+ * @dst:   Destination address, in kernel space. This buffer must be @ksize
+ *         bytes long.
+ * @ksize: Size of @dst struct.
+ * @src:   Source address, in userspace.
+ * @usize: (Alleged) size of @src struct.
+ *
+ * Copies a struct from userspace to kernel space, in a way that guarantees
+ * backwards-compatibility for struct syscall arguments (as long as future
+ * struct extensions are made such that all new fields are *appended* to the
+ * old struct, and zeroed-out new fields have the same meaning as the old
+ * struct).
+ *
+ * @ksize is just sizeof(*dst), and @usize should've been passed by userspace.
+ * The recommended usage is something like the following:
+ *
+ *   SYSCALL_DEFINE2(foobar, const struct foo __user *, uarg, size_t, usize)
+ *   {
+ *      int err;
+ *      struct foo karg = {};
+ *
+ *      err = copy_struct_from_user(&karg, sizeof(karg), uarg, size);
+ *      if (err)
+ *        return err;
+ *
+ *      // ...
+ *   }
+ *
+ * There are three cases to consider:
+ *  * If @usize == @ksize, then it's copied verbatim.
+ *  * If @usize < @ksize, then the userspace has passed an old struct to a
+ *    newer kernel. The rest of the trailing bytes in @dst (@ksize - @usize)
+ *    are to be zero-filled.
+ *  * If @usize > @ksize, then the userspace has passed a new struct to an
+ *    older kernel. The trailing bytes unknown to the kernel (@usize - @ksize)
+ *    are checked to ensure they are zeroed, otherwise -E2BIG is returned.
+ *
+ * Returns (in all cases, some data may have been copied):
+ *  * -E2BIG:  (@usize > @ksize) and there are non-zero trailing bytes in @src.
+ *  * -EFAULT: access to userspace failed.
+ */
+int copy_struct_from_user(void *dst, size_t ksize,
+			  const void __user *src, size_t usize)
+{
+	size_t size = min(ksize, usize);
+	size_t rest = max(ksize, usize) - size;
+
+	/* Deal with trailing bytes. */
+	if (usize < ksize) {
+		memset(dst + size, 0, rest);
+	} else if (usize > ksize) {
+		int ret = is_zeroed_user(src + size, rest);
+		if (ret <= 0)
+			return ret ?: -E2BIG;
+	}
+	/* Copy the interoperable parts of the struct. */
+	if (copy_from_user(dst, src, size))
+		return -EFAULT;
+	return 0;
+}