udp: fix incorrect parameter validation in the udp_lib_getsockopt() function

BugLink: https://bugs.launchpad.net/bugs/2060097

[ Upstream commit 4bb3ba7b74fceec6f558745b25a43c6521cf5506 ]

The 'len' variable can't be negative when assigned the result of
'min_t' because all 'min_t' parameters are cast to unsigned int,
and then the minimum one is chosen.

To fix the logic, check 'len' as read from 'optlen',
where the types of relevant variables are (signed) int.

Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Gavrilov Ilia <Ilia.Gavrilov@infotecs.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 2773d355f05052b0dd43d10d87c9d1c1bbd28dcc)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

net/ipv4/udp.c

@@ -2792,11 +2792,11 @@ int udp_lib_getsockopt(struct sock *sk, int level, int optname,
 	if (get_user(len, optlen))
 		return -EFAULT;
 
-	len = min_t(unsigned int, len, sizeof(int));
-
 	if (len < 0)
 		return -EINVAL;
 
+	len = min_t(unsigned int, len, sizeof(int));
+
 	switch (optname) {
 	case UDP_CORK:
 		val = udp_test_bit(CORK, sk);