tcp: fix 0 divide in __tcp_select_window()

syszkaller fuzzer was able to trigger a divide by zero, when
TCP window scaling is not enabled.

SO_RCVBUF can be used not only to increase sk_rcvbuf, also
to decrease it below current receive buffers utilization.

If mss is negative or 0, just return a zero TCP window.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Dmitry Vyukov  <dvyukov@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 2 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 1d5331a1b1dc..8ce50dc3ab8c 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2518,9 +2518,11 @@ u32 __tcp_select_window(struct sock *sk)
         int full_space = min_t(int, tp->window_clamp, allowed_space);
         int window;
 
-        if (mss > full_space)
+        if (unlikely(mss > full_space)) {
                 mss = full_space;
-
+                if (mss <= 0)
+                        return 0;
+        }
         if (free_space < (full_space >> 1)) {
                 icsk->icsk_ack.quick = 0;