net: avoid signed overflows for SO_{SND|RCV}BUFFORCE

CAP_NET_ADMIN users should not be allowed to set negative
sk_sndbuf or sk_rcvbuf values, as it can lead to various memory
corruptions, crashes, OOM...

Note that before commit 82981930 ("net: cleanups in
sock_setsockopt()"), the bug was even more serious, since SO_SNDBUF
and SO_RCVBUF were vulnerable.

This needs to be backported to all known linux kernels.

Again, many thanks to syzkaller team for discovering this gem.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit b98b0bc8)
CVE-2016-9793
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: John Donnelly <john.donnelly@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Steve Beattie <sbeattie@ubuntu.com>

net/core/sock.c

@@ -745,7 +745,7 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
 		val = min_t(u32, val, sysctl_wmem_max);
 set_sndbuf:
 		sk->sk_userlocks |= SOCK_SNDBUF_LOCK;
-		sk->sk_sndbuf = max_t(u32, val * 2, SOCK_MIN_SNDBUF);
+		sk->sk_sndbuf = max_t(int, val * 2, SOCK_MIN_SNDBUF);
 		/* Wake up sending tasks if we upped the value. */
 		sk->sk_write_space(sk);
 		break;
@@ -781,7 +781,7 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
 		 * returning the value we actually used in getsockopt
 		 * is the most desirable behavior.
 		 */
-		sk->sk_rcvbuf = max_t(u32, val * 2, SOCK_MIN_RCVBUF);
+		sk->sk_rcvbuf = max_t(int, val * 2, SOCK_MIN_RCVBUF);
 		break;
 
 	case SO_RCVBUFFORCE: