net: compat: assert the size of cmsg copied in is as expected

The actual length of cmsg fetched in during the second loop
(i.e., kcmsg - kcmsg_base) could be different from what we
get from the first loop (i.e., kcmlen).

The main reason is that the two get_user() calls in the two
loops (i.e., get_user(ucmlen, &ucmsg->cmsg_len) and
__get_user(ucmlen, &ucmsg->cmsg_len)) could cause ucmlen
to have different values even they fetch from the same userspace
address, as user can race to change the memory content in
&ucmsg->cmsg_len across fetches.

Although in the second loop, the sanity check
if ((char *)kcmsg_base + kcmlen - (char *)kcmsg < CMSG_ALIGN(tmp))
is inplace, it only ensures that the cmsg fetched in during the
second loop does not exceed the length of kcmlen, but not
necessarily equal to kcmlen. But indicated by the assignment
kmsg->msg_controllen = kcmlen, we should enforce that.

This patch adds this additional sanity check and ensures that
what is recorded in kmsg->msg_controllen is the actual cmsg length.
Signed-off-by: Meng Xu <mengxu.gatech@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/compat.c

@@ -185,6 +185,13 @@ int cmsghdr_from_user_compat_to_kern(struct msghdr *kmsg, struct sock *sk,
 		ucmsg = cmsg_compat_nxthdr(kmsg, ucmsg, ucmlen);
 	}
 
+	/*
+	 * check the length of messages copied in is the same as the
+	 * what we get from the first loop
+	 */
+	if ((char *)kcmsg - (char *)kcmsg_base != kcmlen)
+		goto Einval;
+
 	/* Ok, looks like we made it.  Hook it up and return success. */
 	kmsg->msg_control = kcmsg_base;
 	kmsg->msg_controllen = kcmlen;