[PATCH net] tcp: fix 0 divide in __tcp_select_window()

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

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 <eduma...@google.com>
Reported-by: Dmitry Vyukov  <dvyu...@google.com>
---
No Fixes: tag, this is a day-0 bug.

 net/ipv4/tcp_output.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 
1d5331a1b1dc2677316148ba9852c191e7ed0fd4..8ce50dc3ab8cac821b8a2c3e0d31f0aa42f5c9d5
 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;