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>
(cherry picked from commit 9c443ac4559a47ed99859bd80d14dc4b6dd220a1)
(cherry picked from commit cbd8685e82d1b4a7160250845940be68510455ac)

3 files changed, 94 insertions, 22 deletions

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 88fce77349..1dd7356a16 100644
--- a/sysdeps/unix/sysv/linux/bits/socket.h
+++ b/sysdeps/unix/sysv/linux/bits/socket.h
@@ -306,6 +306,12 @@ 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
@@ -315,18 +321,38 @@ 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)