diff mbox

usercopy: Avoid soft lockups in test_check_nonzero_user()

Message ID 20191011022447.24249-1-mpe@ellerman.id.au
State Not applicable
Headers

Commit Message

Michael Ellerman Oct. 11, 2019, 2:24 a.m. UTC 
  On a machine with a 64K PAGE_SIZE, the nested for loops in
test_check_nonzero_user() can lead to soft lockups, eg:

  watchdog: BUG: soft lockup - CPU#4 stuck for 22s! [modprobe:611]
  Modules linked in: test_user_copy(+) vmx_crypto gf128mul crc32c_vpmsum virtio_balloon ip_tables x_tables autofs4
  CPU: 4 PID: 611 Comm: modprobe Tainted: G             L    5.4.0-rc1-gcc-8.2.0-00001-gf5a1a536fa14-dirty #1151
  ...
  NIP __might_sleep+0x20/0xc0
  LR  __might_fault+0x40/0x60
  Call Trace:
    check_zeroed_user+0x12c/0x200
    test_user_copy_init+0x67c/0x1210 [test_user_copy]
    do_one_initcall+0x60/0x340
    do_init_module+0x7c/0x2f0
    load_module+0x2d94/0x30e0
    __do_sys_finit_module+0xc8/0x150
    system_call+0x5c/0x68

Even with a 4K PAGE_SIZE the test takes multiple seconds. Instead
tweak it to only scan a 1024 byte region, but make it cross the
page boundary.

Fixes: f5a1a536fa14 ("lib: introduce copy_struct_from_user() helper")
Suggested-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 lib/test_user_copy.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

How does this look? It runs in < 1s on my machine here.

cheers

Comments

Aleksa Sarai Oct. 11, 2019, 3:48 a.m. UTC | #1 
On 2019-10-11, Michael Ellerman <mpe@ellerman.id.au> wrote:
> On a machine with a 64K PAGE_SIZE, the nested for loops in
> test_check_nonzero_user() can lead to soft lockups, eg:
> 
>   watchdog: BUG: soft lockup - CPU#4 stuck for 22s! [modprobe:611]
>   Modules linked in: test_user_copy(+) vmx_crypto gf128mul crc32c_vpmsum virtio_balloon ip_tables x_tables autofs4
>   CPU: 4 PID: 611 Comm: modprobe Tainted: G             L    5.4.0-rc1-gcc-8.2.0-00001-gf5a1a536fa14-dirty #1151
>   ...
>   NIP __might_sleep+0x20/0xc0
>   LR  __might_fault+0x40/0x60
>   Call Trace:
>     check_zeroed_user+0x12c/0x200
>     test_user_copy_init+0x67c/0x1210 [test_user_copy]
>     do_one_initcall+0x60/0x340
>     do_init_module+0x7c/0x2f0
>     load_module+0x2d94/0x30e0
>     __do_sys_finit_module+0xc8/0x150
>     system_call+0x5c/0x68
> 
> Even with a 4K PAGE_SIZE the test takes multiple seconds. Instead
> tweak it to only scan a 1024 byte region, but make it cross the
> page boundary.
> 
> Fixes: f5a1a536fa14 ("lib: introduce copy_struct_from_user() helper")
> Suggested-by: Aleksa Sarai <cyphar@cyphar.com>
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
>  lib/test_user_copy.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> How does this look? It runs in < 1s on my machine here.
> 
> cheers
> 
> diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> index 950ee88cd6ac..9fb6bc609d4c 100644
> --- a/lib/test_user_copy.c
> +++ b/lib/test_user_copy.c
> @@ -47,9 +47,26 @@ static bool is_zeroed(void *from, size_t size)
>  static int test_check_nonzero_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;
> +	size_t start, end, i, zero_start, zero_end;
> +
> +	if (test(size < 1024, "buffer too small"))
> +		return -EINVAL;
> +
> +	/*
> +	 * We want to cross a page boundary to exercise the code more
> +	 * effectively. We assume the buffer we're passed has a page boundary at
> +	 * size / 2. We also don't want to make the size we scan too large,
> +	 * otherwise the test can take a long time and cause soft lockups. So
> +	 * scan a 1024 byte region across the page boundary.
> +	 */
> +	start = size / 2 - 512;
> +	size = 1024;

I don't think it's necessary to do "size / 2" here -- you can just use
PAGE_SIZE directly and check above that "size == 2*PAGE_SIZE" (not that
this check is exceptionally necessary -- since there's only one caller
of this function and it's in the same file).

> +
> +	kmem += start;
> +	umem += start;
> +
> +	zero_start = size / 4;
> +	zero_end = size - zero_start;
>  
>  	/*
>  	 * We conduct a series of check_nonzero_user() tests on a block of memory
Christian Brauner Oct. 11, 2019, 9:43 a.m. UTC | #2 
On Fri, Oct 11, 2019 at 02:48:10PM +1100, Aleksa Sarai wrote:
> On 2019-10-11, Michael Ellerman <mpe@ellerman.id.au> wrote:
> > On a machine with a 64K PAGE_SIZE, the nested for loops in
> > test_check_nonzero_user() can lead to soft lockups, eg:
> > 
> >   watchdog: BUG: soft lockup - CPU#4 stuck for 22s! [modprobe:611]
> >   Modules linked in: test_user_copy(+) vmx_crypto gf128mul crc32c_vpmsum virtio_balloon ip_tables x_tables autofs4
> >   CPU: 4 PID: 611 Comm: modprobe Tainted: G             L    5.4.0-rc1-gcc-8.2.0-00001-gf5a1a536fa14-dirty #1151
> >   ...
> >   NIP __might_sleep+0x20/0xc0
> >   LR  __might_fault+0x40/0x60
> >   Call Trace:
> >     check_zeroed_user+0x12c/0x200
> >     test_user_copy_init+0x67c/0x1210 [test_user_copy]
> >     do_one_initcall+0x60/0x340
> >     do_init_module+0x7c/0x2f0
> >     load_module+0x2d94/0x30e0
> >     __do_sys_finit_module+0xc8/0x150
> >     system_call+0x5c/0x68
> > 
> > Even with a 4K PAGE_SIZE the test takes multiple seconds. Instead
> > tweak it to only scan a 1024 byte region, but make it cross the
> > page boundary.
> > 
> > Fixes: f5a1a536fa14 ("lib: introduce copy_struct_from_user() helper")
> > Suggested-by: Aleksa Sarai <cyphar@cyphar.com>
> > Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> > ---
> >  lib/test_user_copy.c | 23 ++++++++++++++++++++---
> >  1 file changed, 20 insertions(+), 3 deletions(-)
> > 
> > How does this look? It runs in < 1s on my machine here.
> > 
> > cheers
> > 
> > diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> > index 950ee88cd6ac..9fb6bc609d4c 100644
> > --- a/lib/test_user_copy.c
> > +++ b/lib/test_user_copy.c
> > @@ -47,9 +47,26 @@ static bool is_zeroed(void *from, size_t size)
> >  static int test_check_nonzero_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;
> > +	size_t start, end, i, zero_start, zero_end;
> > +
> > +	if (test(size < 1024, "buffer too small"))
> > +		return -EINVAL;
> > +
> > +	/*
> > +	 * We want to cross a page boundary to exercise the code more
> > +	 * effectively. We assume the buffer we're passed has a page boundary at
> > +	 * size / 2. We also don't want to make the size we scan too large,
> > +	 * otherwise the test can take a long time and cause soft lockups. So
> > +	 * scan a 1024 byte region across the page boundary.
> > +	 */
> > +	start = size / 2 - 512;
> > +	size = 1024;
> 
> I don't think it's necessary to do "size / 2" here -- you can just use
> PAGE_SIZE directly and check above that "size == 2*PAGE_SIZE" (not that
> this check is exceptionally necessary -- since there's only one caller
> of this function and it's in the same file).

Michael, in case you resend, can you make my life a little easier and do
it on top of
https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=copy_struct_from_user
please. I have a fix from Aleksa sitting in there laready that _might_
cause a conflict otherwise.

Christian
Michael Ellerman Oct. 16, 2019, 12:28 p.m. UTC | #3 
Christian Brauner <christian.brauner@ubuntu.com> writes:
> On Fri, Oct 11, 2019 at 02:48:10PM +1100, Aleksa Sarai wrote:
>> On 2019-10-11, Michael Ellerman <mpe@ellerman.id.au> wrote:
>> > On a machine with a 64K PAGE_SIZE, the nested for loops in
>> > test_check_nonzero_user() can lead to soft lockups, eg:
>> > 
>> >   watchdog: BUG: soft lockup - CPU#4 stuck for 22s! [modprobe:611]
>> >   Modules linked in: test_user_copy(+) vmx_crypto gf128mul crc32c_vpmsum virtio_balloon ip_tables x_tables autofs4
>> >   CPU: 4 PID: 611 Comm: modprobe Tainted: G             L    5.4.0-rc1-gcc-8.2.0-00001-gf5a1a536fa14-dirty #1151
>> >   ...
>> >   NIP __might_sleep+0x20/0xc0
>> >   LR  __might_fault+0x40/0x60
>> >   Call Trace:
>> >     check_zeroed_user+0x12c/0x200
>> >     test_user_copy_init+0x67c/0x1210 [test_user_copy]
>> >     do_one_initcall+0x60/0x340
>> >     do_init_module+0x7c/0x2f0
>> >     load_module+0x2d94/0x30e0
>> >     __do_sys_finit_module+0xc8/0x150
>> >     system_call+0x5c/0x68
>> > 
>> > Even with a 4K PAGE_SIZE the test takes multiple seconds. Instead
>> > tweak it to only scan a 1024 byte region, but make it cross the
>> > page boundary.
>> > 
>> > Fixes: f5a1a536fa14 ("lib: introduce copy_struct_from_user() helper")
>> > Suggested-by: Aleksa Sarai <cyphar@cyphar.com>
>> > Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
>> > ---
>> >  lib/test_user_copy.c | 23 ++++++++++++++++++++---
>> >  1 file changed, 20 insertions(+), 3 deletions(-)
>> > 
>> > How does this look? It runs in < 1s on my machine here.
>> > 
>> > cheers
>> > 
>> > diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
>> > index 950ee88cd6ac..9fb6bc609d4c 100644
>> > --- a/lib/test_user_copy.c
>> > +++ b/lib/test_user_copy.c
>> > @@ -47,9 +47,26 @@ static bool is_zeroed(void *from, size_t size)
>> >  static int test_check_nonzero_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;
>> > +	size_t start, end, i, zero_start, zero_end;
>> > +
>> > +	if (test(size < 1024, "buffer too small"))
>> > +		return -EINVAL;
>> > +
>> > +	/*
>> > +	 * We want to cross a page boundary to exercise the code more
>> > +	 * effectively. We assume the buffer we're passed has a page boundary at
>> > +	 * size / 2. We also don't want to make the size we scan too large,
>> > +	 * otherwise the test can take a long time and cause soft lockups. So
>> > +	 * scan a 1024 byte region across the page boundary.
>> > +	 */
>> > +	start = size / 2 - 512;
>> > +	size = 1024;
>> 
>> I don't think it's necessary to do "size / 2" here -- you can just use
>> PAGE_SIZE directly and check above that "size == 2*PAGE_SIZE" (not that
>> this check is exceptionally necessary -- since there's only one caller
>> of this function and it's in the same file).
>
> Michael, in case you resend, can you make my life a little easier and do
> it on top of
> https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=copy_struct_from_user
> please. I have a fix from Aleksa sitting in there laready that _might_
> cause a conflict otherwise.

No worries, done.

cheers
Christian Brauner Oct. 16, 2019, 12:45 p.m. UTC | #4 
On Wed, Oct 16, 2019 at 11:28:20PM +1100, Michael Ellerman wrote:
> Christian Brauner <christian.brauner@ubuntu.com> writes:
> > On Fri, Oct 11, 2019 at 02:48:10PM +1100, Aleksa Sarai wrote:
> >> On 2019-10-11, Michael Ellerman <mpe@ellerman.id.au> wrote:
> >> > On a machine with a 64K PAGE_SIZE, the nested for loops in
> >> > test_check_nonzero_user() can lead to soft lockups, eg:
> >> > 
> >> >   watchdog: BUG: soft lockup - CPU#4 stuck for 22s! [modprobe:611]
> >> >   Modules linked in: test_user_copy(+) vmx_crypto gf128mul crc32c_vpmsum virtio_balloon ip_tables x_tables autofs4
> >> >   CPU: 4 PID: 611 Comm: modprobe Tainted: G             L    5.4.0-rc1-gcc-8.2.0-00001-gf5a1a536fa14-dirty #1151
> >> >   ...
> >> >   NIP __might_sleep+0x20/0xc0
> >> >   LR  __might_fault+0x40/0x60
> >> >   Call Trace:
> >> >     check_zeroed_user+0x12c/0x200
> >> >     test_user_copy_init+0x67c/0x1210 [test_user_copy]
> >> >     do_one_initcall+0x60/0x340
> >> >     do_init_module+0x7c/0x2f0
> >> >     load_module+0x2d94/0x30e0
> >> >     __do_sys_finit_module+0xc8/0x150
> >> >     system_call+0x5c/0x68
> >> > 
> >> > Even with a 4K PAGE_SIZE the test takes multiple seconds. Instead
> >> > tweak it to only scan a 1024 byte region, but make it cross the
> >> > page boundary.
> >> > 
> >> > Fixes: f5a1a536fa14 ("lib: introduce copy_struct_from_user() helper")
> >> > Suggested-by: Aleksa Sarai <cyphar@cyphar.com>
> >> > Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> >> > ---
> >> >  lib/test_user_copy.c | 23 ++++++++++++++++++++---
> >> >  1 file changed, 20 insertions(+), 3 deletions(-)
> >> > 
> >> > How does this look? It runs in < 1s on my machine here.
> >> > 
> >> > cheers
> >> > 
> >> > diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> >> > index 950ee88cd6ac..9fb6bc609d4c 100644
> >> > --- a/lib/test_user_copy.c
> >> > +++ b/lib/test_user_copy.c
> >> > @@ -47,9 +47,26 @@ static bool is_zeroed(void *from, size_t size)
> >> >  static int test_check_nonzero_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;
> >> > +	size_t start, end, i, zero_start, zero_end;
> >> > +
> >> > +	if (test(size < 1024, "buffer too small"))
> >> > +		return -EINVAL;
> >> > +
> >> > +	/*
> >> > +	 * We want to cross a page boundary to exercise the code more
> >> > +	 * effectively. We assume the buffer we're passed has a page boundary at
> >> > +	 * size / 2. We also don't want to make the size we scan too large,
> >> > +	 * otherwise the test can take a long time and cause soft lockups. So
> >> > +	 * scan a 1024 byte region across the page boundary.
> >> > +	 */
> >> > +	start = size / 2 - 512;
> >> > +	size = 1024;
> >> 
> >> I don't think it's necessary to do "size / 2" here -- you can just use
> >> PAGE_SIZE directly and check above that "size == 2*PAGE_SIZE" (not that
> >> this check is exceptionally necessary -- since there's only one caller
> >> of this function and it's in the same file).
> >
> > Michael, in case you resend, can you make my life a little easier and do
> > it on top of
> > https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=copy_struct_from_user
> > please. I have a fix from Aleksa sitting in there laready that _might_
> > cause a conflict otherwise.
> 
> No worries, done.

Thank you!
Christian
diff mbox

Patch

diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
index 950ee88cd6ac..9fb6bc609d4c 100644
--- a/lib/test_user_copy.c
+++ b/lib/test_user_copy.c
@@ -47,9 +47,26 @@  static bool is_zeroed(void *from, size_t size)
 static int test_check_nonzero_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;
+	size_t start, end, i, zero_start, zero_end;
+
+	if (test(size < 1024, "buffer too small"))
+		return -EINVAL;
+
+	/*
+	 * We want to cross a page boundary to exercise the code more
+	 * effectively. We assume the buffer we're passed has a page boundary at
+	 * size / 2. We also don't want to make the size we scan too large,
+	 * otherwise the test can take a long time and cause soft lockups. So
+	 * scan a 1024 byte region across the page boundary.
+	 */
+	start = size / 2 - 512;
+	size = 1024;
+
+	kmem += start;
+	umem += start;
+
+	zero_start = size / 4;
+	zero_end = size - zero_start;
 
 	/*
 	 * We conduct a series of check_nonzero_user() tests on a block of memory