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]

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

1 files changed, 28 insertions, 8 deletions

diff --git a/sysdeps/unix/sysv/linux/cmsg_nxthdr.c b/sysdeps/unix/sysv/linux/cmsg_nxthdr.c
index 15b7a3a..24f72b7 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)