linux-kernel - [PATCH 3.12 052/113] tcp: fix 0 divide in __tcp_select

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Message-Id: <c48ae3c2a1ffc24d7a4e6244c214ac744bcd8ff0.1488791430.git.jslaby@suse.cz>
Date:   Mon,  6 Mar 2017 10:11:16 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     stable@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 052/113] tcp: fix 0 divide in __tcp_select_window()

From: Eric Dumazet <edumazet@...gle.com>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

[ Upstream commit 06425c308b92eaf60767bc71d359f4cbc7a561f8 ]

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@...gle.com>
Reported-by: Dmitry Vyukov  <dvyukov@...gle.com>
Acked-by: Neal Cardwell <ncardwell@...gle.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 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 c807d5790ca1..d92c4b69f7ea 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2163,9 +2163,11 @@ u32 __tcp_select_window(struct sock *sk)
 	int full_space = min_t(int, tp->window_clamp, tcp_full_space(sk));
 	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;
 
-- 
2.12.0