tcp: Fix sign comparison bug in getsockopt(TCP_ZEROCOPY_RECEIVE)

getsockopt(TCP_ZEROCOPY_RECEIVE) has a bug where we read a
user-provided "len" field of type signed int, and then compare the
value to the result of an "offsetofend" operation, which is unsigned.

Negative values provided by the user will be promoted to large
positive numbers; thus checking that len < offsetofend() will return
false when the intention was that it return true.

Note that while len is originally checked for negative values earlier
on in do_tcp_getsockopt(), subsequent calls to get_user() re-read the
value from userspace which may have changed in the meantime.

Therefore, re-add the check for negative values after the call to
get_user in the handler code for TCP_ZEROCOPY_RECEIVE.

Fixes: c8856c05 ("tcp-zerocopy: Return inq along with tcp receive zerocopy.")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Arjun Roy <arjunroy@google.com>
Link: https://lore.kernel.org/r/20210225232628.4033281-1-arjunroy.kdev@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp.c

@@ -4143,7 +4143,8 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
 
 		if (get_user(len, optlen))
 			return -EFAULT;
-		if (len < offsetofend(struct tcp_zerocopy_receive, length))
+		if (len < 0 ||
+		    len < offsetofend(struct tcp_zerocopy_receive, length))
 			return -EINVAL;
 		if (unlikely(len > sizeof(zc))) {
 			err = check_zeroed_user(optval + sizeof(zc),