netlink: fix (theoretical) overrun in message iteration

See commit 1045b03e07 ("netlink: fix
overrun in attribute iteration") for a detailed explanation of why
this patch is necessary.

In short, nlmsg_next() can make "remaining" go negative, and the
remaining >= sizeof(...) comparison will promote "remaining" to an
unsigned type, which means that the expression will evaluate to
true for negative numbers, even though it was not intended.

I put "theoretical" in the title because I have no evidence that
this can actually happen, but I suspect that a crafted netlink
packet can trigger some badness.

Note that the last test, which seemingly has the exact same
problem (also true for nla_ok()), is perfectly OK, since we
already know that remaining is positive.

Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netlink.h

@@ -332,7 +332,7 @@ static inline int nlmsg_attrlen(const struct nlmsghdr *nlh, int hdrlen)
  */
 static inline int nlmsg_ok(const struct nlmsghdr *nlh, int remaining)
 {
-	return (remaining >= sizeof(struct nlmsghdr) &&
+	return (remaining >= (int) sizeof(struct nlmsghdr) &&
 		nlh->nlmsg_len >= sizeof(struct nlmsghdr) &&
 		nlh->nlmsg_len <= remaining);
 }