[v4] socket: Check lengths before advancing pointer in CMSG_NXTHDR

  The inline and library functions that the CMSG_NXTHDR macro may expand
to increment the pointer to the header before checking the stride of
the increment against available space.  Since C only allows incrementing
pointers to one past the end of an array, the increment must be done
after a length check.  This commit fixes that and includes a regression
test for CMSG_FIRSTHDR and CMSG_NXTHDR.

The Linux, Hurd, and generic headers are all changed.

Tested on Linux on armv7hl, i686, x86_64, aarch64, ppc64le, and s390x.

[BZ #28846]
---
v3: https://sourceware.org/pipermail/libc-alpha/2022-July/140854.html

Notes on v4:

* Addressed review comments from Siddhesh:

1. (sizeof (struct cmsghdr) + __CMSG_PADDING (cmsg_len)):
   defined as size_needed.

2.
> OK, but I wonder if there's utility in making the padding a generic 
> macro, e.g.

> #define ALIGN_PADDING(n, a) ((a - (n & (a - 1))) & (a - 1))

This sounds useful, and actually it would be great to move *all* of the
duplicate code between these versions into a separate file and include it
in these variants.  I'll try to do a follow-up with this soon. I'm going to
note it down in my TODO.

3.
> __msg_control_ptr doesn't really need the __ since it's a local variable.

I thought so too.  But Florian pointed out that it would interfere with
things like users #define'ing msg_control_ptr before including socket.h.
---
 bits/socket.h                         | 40 ++++++++++--
 socket/Makefile                       |  1 +
 socket/tst-cmsghdr-skeleton.c         | 92 +++++++++++++++++++++++++++
 socket/tst-cmsghdr.c                  | 56 ++++++++++++++++
 sysdeps/mach/hurd/bits/socket.h       | 40 ++++++++++--
 sysdeps/unix/sysv/linux/bits/socket.h | 40 ++++++++++--
 sysdeps/unix/sysv/linux/cmsg_nxthdr.c | 36 ++++++++---
 7 files changed, 276 insertions(+), 29 deletions(-)
 create mode 100644 socket/tst-cmsghdr-skeleton.c
 create mode 100644 socket/tst-cmsghdr.c
  

On 2022-07-29 09:26, Arjun Shankar wrote:
> The inline and library functions that the CMSG_NXTHDR macro may expand
> to increment the pointer to the header before checking the stride of
> the increment against available space.  Since C only allows incrementing
> pointers to one past the end of an array, the increment must be done
> after a length check.  This commit fixes that and includes a regression
> test for CMSG_FIRSTHDR and CMSG_NXTHDR.
> 
> The Linux, Hurd, and generic headers are all changed.
> 
> Tested on Linux on armv7hl, i686, x86_64, aarch64, ppc64le, and s390x.
> 
> [BZ #28846]
> ---
> v3: https://sourceware.org/pipermail/libc-alpha/2022-July/140854.html
> 
> Notes on v4:
> 
> * Addressed review comments from Siddhesh:
> 
> 1. (sizeof (struct cmsghdr) + __CMSG_PADDING (cmsg_len)):
>     defined as size_needed.
> 
> 2.
>> OK, but I wonder if there's utility in making the padding a generic
>> macro, e.g.
> 
>> #define ALIGN_PADDING(n, a) ((a - (n & (a - 1))) & (a - 1))
> 
> This sounds useful, and actually it would be great to move *all* of the
> duplicate code between these versions into a separate file and include it
> in these variants.  I'll try to do a follow-up with this soon. I'm going to
> note it down in my TODO.
> 
> 3.
>> __msg_control_ptr doesn't really need the __ since it's a local variable.
> 
> I thought so too.  But Florian pointed out that it would interfere with
> things like users #define'ing msg_control_ptr before including socket.h.
> ---
>   bits/socket.h                         | 40 ++++++++++--
>   socket/Makefile                       |  1 +
>   socket/tst-cmsghdr-skeleton.c         | 92 +++++++++++++++++++++++++++
>   socket/tst-cmsghdr.c                  | 56 ++++++++++++++++
>   sysdeps/mach/hurd/bits/socket.h       | 40 ++++++++++--
>   sysdeps/unix/sysv/linux/bits/socket.h | 40 ++++++++++--
>   sysdeps/unix/sysv/linux/cmsg_nxthdr.c | 36 ++++++++---
>   7 files changed, 276 insertions(+), 29 deletions(-)
>   create mode 100644 socket/tst-cmsghdr-skeleton.c
>   create mode 100644 socket/tst-cmsghdr.c

LGTM.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

> 
> diff --git a/bits/socket.h b/bits/socket.h
> index 2b99dea33b..aac8c49b00 100644
> --- a/bits/socket.h
> +++ b/bits/socket.h
> @@ -245,6 +245,12 @@ struct cmsghdr
>   			 + CMSG_ALIGN (sizeof (struct cmsghdr)))
>   #define CMSG_LEN(len)   (CMSG_ALIGN (sizeof (struct cmsghdr)) + (len))
>   
> +/* Given a length, return the additional padding necessary such that
> +   len + __CMSG_PADDING(len) == CMSG_ALIGN (len).  */
> +#define __CMSG_PADDING(len) ((sizeof (size_t) \
> +                              - ((len) & (sizeof (size_t) - 1))) \
> +                             & (sizeof (size_t) - 1))
> +
>   extern struct cmsghdr *__cmsg_nxthdr (struct msghdr *__mhdr,
>   				      struct cmsghdr *__cmsg) __THROW;
>   #ifdef __USE_EXTERN_INLINES
> @@ -254,18 +260,38 @@ extern struct cmsghdr *__cmsg_nxthdr (struct msghdr *__mhdr,
>   _EXTERN_INLINE struct cmsghdr *
>   __NTH (__cmsg_nxthdr (struct msghdr *__mhdr, struct cmsghdr *__cmsg))
>   {
> +  /* We may safely assume that __cmsg lies between __mhdr->msg_control and
> +     __mhdr->msg_controllen because the user is required to obtain the first
> +     cmsg via CMSG_FIRSTHDR, set its length, then obtain subsequent cmsgs
> +     via CMSG_NXTHDR, setting lengths along the way.  However, we don't yet
> +     trust the value of __cmsg->cmsg_len and therefore do not use it in any
> +     pointer arithmetic until we check its value.  */
> +
> +  unsigned char * __msg_control_ptr = (unsigned char *) __mhdr->msg_control;
> +  unsigned char * __cmsg_ptr = (unsigned char *) __cmsg;
> +
> +  size_t __size_needed = sizeof (struct cmsghdr)
> +                         + __CMSG_PADDING (__cmsg->cmsg_len);
> +
> +  /* The current header is malformed, too small to be a full header.  */
>     if ((size_t) __cmsg->cmsg_len < sizeof (struct cmsghdr))
> -    /* The kernel header does this so there may be a reason.  */
>       return (struct cmsghdr *) 0;
>   
> +  /* There isn't enough space between __cmsg and the end of the buffer to
> +  hold the current cmsg *and* the next one.  */
> +  if (((size_t)
> +         (__msg_control_ptr + __mhdr->msg_controllen - __cmsg_ptr)
> +       < __size_needed)
> +      || ((size_t)
> +            (__msg_control_ptr + __mhdr->msg_controllen - __cmsg_ptr
> +             - __size_needed)
> +          < __cmsg->cmsg_len))
> +
> +    return (struct cmsghdr *) 0;
> +
> +  /* Now, we trust cmsg_len and can use it to find the next header.  */
>     __cmsg = (struct cmsghdr *) ((unsigned char *) __cmsg
>   			       + CMSG_ALIGN (__cmsg->cmsg_len));
> -  if ((unsigned char *) (__cmsg + 1) > ((unsigned char *) __mhdr->msg_control
> -					+ __mhdr->msg_controllen)
> -      || ((unsigned char *) __cmsg + CMSG_ALIGN (__cmsg->cmsg_len)
> -	  > ((unsigned char *) __mhdr->msg_control + __mhdr->msg_controllen)))
> -    /* No more entries.  */
> -    return (struct cmsghdr *) 0;
>     return __cmsg;
>   }
>   #endif	/* Use `extern inline'.  */
> diff --git a/socket/Makefile b/socket/Makefile
> index 156eec6c85..2bde78387f 100644
> --- a/socket/Makefile
> +++ b/socket/Makefile
> @@ -34,6 +34,7 @@ routines := accept bind connect getpeername getsockname getsockopt	\
>   tests := \
>     tst-accept4 \
>     tst-sockopt \
> +  tst-cmsghdr \
>     # tests
>   
>   tests-internal := \
> diff --git a/socket/tst-cmsghdr-skeleton.c b/socket/tst-cmsghdr-skeleton.c
> new file mode 100644
> index 0000000000..4c6898569b
> --- /dev/null
> +++ b/socket/tst-cmsghdr-skeleton.c
> @@ -0,0 +1,92 @@
> +/* Test ancillary data header creation.
> +   Copyright (C) 2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +/* We use the preprocessor to generate the function/macro tests instead of
> +   using indirection because having all the macro expansions alongside
> +   each other lets the compiler warn us about suspicious pointer
> +   arithmetic across subsequent CMSG_{FIRST,NXT}HDR expansions.  */
> +
> +#include <stdint.h>
> +
> +#define RUN_TEST_CONCAT(suffix) run_test_##suffix
> +#define RUN_TEST_FUNCNAME(suffix) RUN_TEST_CONCAT (suffix)
> +
> +static void
> +RUN_TEST_FUNCNAME (CMSG_NXTHDR_IMPL) (void)
> +{
> +  struct msghdr m = {0};
> +  struct cmsghdr *cmsg;
> +  char cmsgbuf[3 * CMSG_SPACE (sizeof (PAYLOAD))] = {0};
> +
> +  m.msg_control = cmsgbuf;
> +  m.msg_controllen = sizeof (cmsgbuf);
> +
> +  /* First header should point to the start of the buffer.  */
> +  cmsg = CMSG_FIRSTHDR (&m);
> +  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
> +
> +  /* If the first header length consumes the entire buffer, there is no
> +     space remaining for additional headers.  */
> +  cmsg->cmsg_len = sizeof (cmsgbuf);
> +  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
> +  TEST_VERIFY_EXIT (cmsg == NULL);
> +
> +  /* The first header length is so big, using it would cause an overflow.  */
> +  cmsg = CMSG_FIRSTHDR (&m);
> +  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
> +  cmsg->cmsg_len = SIZE_MAX;
> +  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
> +  TEST_VERIFY_EXIT (cmsg == NULL);
> +
> +  /* The first header leaves just enough space to hold another header.  */
> +  cmsg = CMSG_FIRSTHDR (&m);
> +  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
> +  cmsg->cmsg_len = sizeof (cmsgbuf) - sizeof (struct cmsghdr);
> +  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
> +  TEST_VERIFY_EXIT (cmsg != NULL);
> +
> +  /* The first header leaves space but not enough for another header.  */
> +  cmsg = CMSG_FIRSTHDR (&m);
> +  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
> +  cmsg->cmsg_len ++;
> +  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
> +  TEST_VERIFY_EXIT (cmsg == NULL);
> +
> +  /* The second header leaves just enough space to hold another header.  */
> +  cmsg = CMSG_FIRSTHDR (&m);
> +  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
> +  cmsg->cmsg_len = CMSG_LEN (sizeof (PAYLOAD));
> +  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
> +  TEST_VERIFY_EXIT (cmsg != NULL);
> +  cmsg->cmsg_len = sizeof (cmsgbuf)
> +                   - CMSG_SPACE (sizeof (PAYLOAD)) /* First header.  */
> +                   - sizeof (struct cmsghdr);
> +  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
> +  TEST_VERIFY_EXIT (cmsg != NULL);
> +
> +  /* The second header leaves space but not enough for another header.  */
> +  cmsg = CMSG_FIRSTHDR (&m);
> +  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
> +  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
> +  TEST_VERIFY_EXIT (cmsg != NULL);
> +  cmsg->cmsg_len ++;
> +  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
> +  TEST_VERIFY_EXIT (cmsg == NULL);
> +
> +  return;
> +}
> diff --git a/socket/tst-cmsghdr.c b/socket/tst-cmsghdr.c
> new file mode 100644
> index 0000000000..68c96d3c9d
> --- /dev/null
> +++ b/socket/tst-cmsghdr.c
> @@ -0,0 +1,56 @@
> +/* Test ancillary data header creation.
> +   Copyright (C) 2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <sys/socket.h>
> +#include <gnu/lib-names.h>
> +#include <support/xdlfcn.h>
> +#include <support/check.h>
> +
> +#define PAYLOAD "Hello, World!"
> +
> +/* CMSG_NXTHDR is a macro that calls an inline function defined in
> +   bits/socket.h.  In case the function cannot be inlined, libc.so carries
> +   a copy.  Both versions need to be tested.  */
> +
> +#define CMSG_NXTHDR_IMPL CMSG_NXTHDR
> +#include "tst-cmsghdr-skeleton.c"
> +#undef CMSG_NXTHDR_IMPL
> +
> +static struct cmsghdr * (* cmsg_nxthdr) (struct msghdr *, struct cmsghdr *);
> +
> +#define CMSG_NXTHDR_IMPL cmsg_nxthdr
> +#include "tst-cmsghdr-skeleton.c"
> +#undef CMSG_NXTHDR_IMPL
> +
> +static int
> +do_test (void)
> +{
> +  static void *handle;
> +
> +  run_test_CMSG_NXTHDR ();
> +
> +  handle = xdlopen (LIBC_SO, RTLD_LAZY);
> +  cmsg_nxthdr = (struct cmsghdr * (*) (struct msghdr *, struct cmsghdr *))
> +                  xdlsym (handle, "__cmsg_nxthdr");
> +
> +  run_test_cmsg_nxthdr ();
> +
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>
> diff --git a/sysdeps/mach/hurd/bits/socket.h b/sysdeps/mach/hurd/bits/socket.h
> index 5b35ea81ec..70fce4fb27 100644
> --- a/sysdeps/mach/hurd/bits/socket.h
> +++ b/sysdeps/mach/hurd/bits/socket.h
> @@ -249,6 +249,12 @@ struct cmsghdr
>   			 + CMSG_ALIGN (sizeof (struct cmsghdr)))
>   #define CMSG_LEN(len)   (CMSG_ALIGN (sizeof (struct cmsghdr)) + (len))
>   
> +/* Given a length, return the additional padding necessary such that
> +   len + __CMSG_PADDING(len) == CMSG_ALIGN (len).  */
> +#define __CMSG_PADDING(len) ((sizeof (size_t) \
> +                              - ((len) & (sizeof (size_t) - 1))) \
> +                             & (sizeof (size_t) - 1))
> +
>   extern struct cmsghdr *__cmsg_nxthdr (struct msghdr *__mhdr,
>   				      struct cmsghdr *__cmsg) __THROW;
>   #ifdef __USE_EXTERN_INLINES
> @@ -258,18 +264,38 @@ extern struct cmsghdr *__cmsg_nxthdr (struct msghdr *__mhdr,
>   _EXTERN_INLINE struct cmsghdr *
>   __NTH (__cmsg_nxthdr (struct msghdr *__mhdr, struct cmsghdr *__cmsg))
>   {
> +  /* We may safely assume that __cmsg lies between __mhdr->msg_control and
> +     __mhdr->msg_controllen because the user is required to obtain the first
> +     cmsg via CMSG_FIRSTHDR, set its length, then obtain subsequent cmsgs
> +     via CMSG_NXTHDR, setting lengths along the way.  However, we don't yet
> +     trust the value of __cmsg->cmsg_len and therefore do not use it in any
> +     pointer arithmetic until we check its value.  */
> +
> +  unsigned char * __msg_control_ptr = (unsigned char *) __mhdr->msg_control;
> +  unsigned char * __cmsg_ptr = (unsigned char *) __cmsg;
> +
> +  size_t __size_needed = sizeof (struct cmsghdr)
> +                         + __CMSG_PADDING (__cmsg->cmsg_len);
> +
> +  /* The current header is malformed, too small to be a full header.  */
>     if ((size_t) __cmsg->cmsg_len < sizeof (struct cmsghdr))
> -    /* The kernel header does this so there may be a reason.  */
>       return (struct cmsghdr *) 0;
>   
> +  /* There isn't enough space between __cmsg and the end of the buffer to
> +  hold the current cmsg *and* the next one.  */
> +  if (((size_t)
> +         (__msg_control_ptr + __mhdr->msg_controllen - __cmsg_ptr)
> +       < __size_needed)
> +      || ((size_t)
> +            (__msg_control_ptr + __mhdr->msg_controllen - __cmsg_ptr
> +             - __size_needed)
> +          < __cmsg->cmsg_len))
> +
> +    return (struct cmsghdr *) 0;
> +
> +  /* Now, we trust cmsg_len and can use it to find the next header.  */
>     __cmsg = (struct cmsghdr *) ((unsigned char *) __cmsg
>   			       + CMSG_ALIGN (__cmsg->cmsg_len));
> -  if ((unsigned char *) (__cmsg + 1) > ((unsigned char *) __mhdr->msg_control
> -					+ __mhdr->msg_controllen)
> -      || ((unsigned char *) __cmsg + CMSG_ALIGN (__cmsg->cmsg_len)
> -	  > ((unsigned char *) __mhdr->msg_control + __mhdr->msg_controllen)))
> -    /* No more entries.  */
> -    return (struct cmsghdr *) 0;
>     return __cmsg;
>   }
>   #endif	/* Use `extern inline'.  */
> diff --git a/sysdeps/unix/sysv/linux/bits/socket.h b/sysdeps/unix/sysv/linux/bits/socket.h
> index 4f1f810ea1..539b8d7716 100644
> --- a/sysdeps/unix/sysv/linux/bits/socket.h
> +++ b/sysdeps/unix/sysv/linux/bits/socket.h
> @@ -307,6 +307,12 @@ struct cmsghdr
>   			 + CMSG_ALIGN (sizeof (struct cmsghdr)))
>   #define CMSG_LEN(len)   (CMSG_ALIGN (sizeof (struct cmsghdr)) + (len))
>   
> +/* Given a length, return the additional padding necessary such that
> +   len + __CMSG_PADDING(len) == CMSG_ALIGN (len).  */
> +#define __CMSG_PADDING(len) ((sizeof (size_t) \
> +                              - ((len) & (sizeof (size_t) - 1))) \
> +                             & (sizeof (size_t) - 1))
> +
>   extern struct cmsghdr *__cmsg_nxthdr (struct msghdr *__mhdr,
>   				      struct cmsghdr *__cmsg) __THROW;
>   #ifdef __USE_EXTERN_INLINES
> @@ -316,18 +322,38 @@ extern struct cmsghdr *__cmsg_nxthdr (struct msghdr *__mhdr,
>   _EXTERN_INLINE struct cmsghdr *
>   __NTH (__cmsg_nxthdr (struct msghdr *__mhdr, struct cmsghdr *__cmsg))
>   {
> +  /* We may safely assume that __cmsg lies between __mhdr->msg_control and
> +     __mhdr->msg_controllen because the user is required to obtain the first
> +     cmsg via CMSG_FIRSTHDR, set its length, then obtain subsequent cmsgs
> +     via CMSG_NXTHDR, setting lengths along the way.  However, we don't yet
> +     trust the value of __cmsg->cmsg_len and therefore do not use it in any
> +     pointer arithmetic until we check its value.  */
> +
> +  unsigned char * __msg_control_ptr = (unsigned char *) __mhdr->msg_control;
> +  unsigned char * __cmsg_ptr = (unsigned char *) __cmsg;
> +
> +  size_t __size_needed = sizeof (struct cmsghdr)
> +                         + __CMSG_PADDING (__cmsg->cmsg_len);
> +
> +  /* The current header is malformed, too small to be a full header.  */
>     if ((size_t) __cmsg->cmsg_len < sizeof (struct cmsghdr))
> -    /* The kernel header does this so there may be a reason.  */
>       return (struct cmsghdr *) 0;
>   
> +  /* There isn't enough space between __cmsg and the end of the buffer to
> +  hold the current cmsg *and* the next one.  */
> +  if (((size_t)
> +         (__msg_control_ptr + __mhdr->msg_controllen - __cmsg_ptr)
> +       < __size_needed)
> +      || ((size_t)
> +            (__msg_control_ptr + __mhdr->msg_controllen - __cmsg_ptr
> +             - __size_needed)
> +          < __cmsg->cmsg_len))
> +
> +    return (struct cmsghdr *) 0;
> +
> +  /* Now, we trust cmsg_len and can use it to find the next header.  */
>     __cmsg = (struct cmsghdr *) ((unsigned char *) __cmsg
>   			       + CMSG_ALIGN (__cmsg->cmsg_len));
> -  if ((unsigned char *) (__cmsg + 1) > ((unsigned char *) __mhdr->msg_control
> -					+ __mhdr->msg_controllen)
> -      || ((unsigned char *) __cmsg + CMSG_ALIGN (__cmsg->cmsg_len)
> -	  > ((unsigned char *) __mhdr->msg_control + __mhdr->msg_controllen)))
> -    /* No more entries.  */
> -    return (struct cmsghdr *) 0;
>     return __cmsg;
>   }
>   #endif	/* Use `extern inline'.  */
> diff --git a/sysdeps/unix/sysv/linux/cmsg_nxthdr.c b/sysdeps/unix/sysv/linux/cmsg_nxthdr.c
> index 15b7a3a925..24f72b797a 100644
> --- a/sysdeps/unix/sysv/linux/cmsg_nxthdr.c
> +++ b/sysdeps/unix/sysv/linux/cmsg_nxthdr.c
> @@ -23,18 +23,38 @@
>   struct cmsghdr *
>   __cmsg_nxthdr (struct msghdr *mhdr, struct cmsghdr *cmsg)
>   {
> +  /* We may safely assume that cmsg lies between mhdr->msg_control and
> +     mhdr->msg_controllen because the user is required to obtain the first
> +     cmsg via CMSG_FIRSTHDR, set its length, then obtain subsequent cmsgs
> +     via CMSG_NXTHDR, setting lengths along the way.  However, we don't yet
> +     trust the value of cmsg->cmsg_len and therefore do not use it in any
> +     pointer arithmetic until we check its value.  */
> +
> +  unsigned char * msg_control_ptr = (unsigned char *) mhdr->msg_control;
> +  unsigned char * cmsg_ptr = (unsigned char *) cmsg;
> +
> +  size_t size_needed = sizeof (struct cmsghdr)
> +                       + __CMSG_PADDING (cmsg->cmsg_len);
> +
> +  /* The current header is malformed, too small to be a full header.  */
>     if ((size_t) cmsg->cmsg_len < sizeof (struct cmsghdr))
> -    /* The kernel header does this so there may be a reason.  */
> -    return NULL;
> +    return (struct cmsghdr *) 0;
> +
> +  /* There isn't enough space between cmsg and the end of the buffer to
> +  hold the current cmsg *and* the next one.  */
> +  if (((size_t)
> +         (msg_control_ptr + mhdr->msg_controllen - cmsg_ptr)
> +       < size_needed)
> +      || ((size_t)
> +            (msg_control_ptr + mhdr->msg_controllen - cmsg_ptr
> +             - size_needed)
> +          < cmsg->cmsg_len))
> +
> +    return (struct cmsghdr *) 0;
>   
> +  /* Now, we trust cmsg_len and can use it to find the next header.  */
>     cmsg = (struct cmsghdr *) ((unsigned char *) cmsg
>   			     + CMSG_ALIGN (cmsg->cmsg_len));
> -  if ((unsigned char *) (cmsg + 1) > ((unsigned char *) mhdr->msg_control
> -				      + mhdr->msg_controllen)
> -      || ((unsigned char *) cmsg + CMSG_ALIGN (cmsg->cmsg_len)
> -	  > ((unsigned char *) mhdr->msg_control + mhdr->msg_controllen)))
> -    /* No more entries.  */
> -    return NULL;
>     return cmsg;
>   }
>   libc_hidden_def (__cmsg_nxthdr)
  

On 7/29/22 12:40, Siddhesh Poyarekar wrote:
> On 2022-07-29 09:26, Arjun Shankar wrote:
>> The inline and library functions that the CMSG_NXTHDR macro may expand
>> to increment the pointer to the header before checking the stride of
>> the increment against available space.  Since C only allows incrementing
>> pointers to one past the end of an array, the increment must be done
>> after a length check.  This commit fixes that and includes a regression
>> test for CMSG_FIRSTHDR and CMSG_NXTHDR.
>>
>> The Linux, Hurd, and generic headers are all changed.
>>
>> Tested on Linux on armv7hl, i686, x86_64, aarch64, ppc64le, and s390x.
>>
>> [BZ #28846]
>> ---
>> v3: https://sourceware.org/pipermail/libc-alpha/2022-July/140854.html
>>
>> Notes on v4:
>>
>> * Addressed review comments from Siddhesh:
>>
>> 1. (sizeof (struct cmsghdr) + __CMSG_PADDING (cmsg_len)):
>>     defined as size_needed.
>>
>> 2.
>>> OK, but I wonder if there's utility in making the padding a generic
>>> macro, e.g.
>>
>>> #define ALIGN_PADDING(n, a) ((a - (n & (a - 1))) & (a - 1))
>>
>> This sounds useful, and actually it would be great to move *all* of the
>> duplicate code between these versions into a separate file and include it
>> in these variants.  I'll try to do a follow-up with this soon. I'm going to
>> note it down in my TODO.
>>
>> 3.
>>> __msg_control_ptr doesn't really need the __ since it's a local variable.
>>
>> I thought so too.  But Florian pointed out that it would interfere with
>> things like users #define'ing msg_control_ptr before including socket.h.
>> ---
>>   bits/socket.h                         | 40 ++++++++++--
>>   socket/Makefile                       |  1 +
>>   socket/tst-cmsghdr-skeleton.c         | 92 +++++++++++++++++++++++++++
>>   socket/tst-cmsghdr.c                  | 56 ++++++++++++++++
>>   sysdeps/mach/hurd/bits/socket.h       | 40 ++++++++++--
>>   sysdeps/unix/sysv/linux/bits/socket.h | 40 ++++++++++--
>>   sysdeps/unix/sysv/linux/cmsg_nxthdr.c | 36 ++++++++---
>>   7 files changed, 276 insertions(+), 29 deletions(-)
>>   create mode 100644 socket/tst-cmsghdr-skeleton.c
>>   create mode 100644 socket/tst-cmsghdr.c
> 
> LGTM.
> 
> Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

Just a reminder that the branch is frozen for glibc 2.36 release.

This is good for 2.37 when it opens.