linux-kernel - [PATCH AUTOSEL for 4.9 066/219] tcp: sysctl: Fix a race to avoid unexpected 0 window from space

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Date:   Sat, 3 Mar 2018 22:28:34 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
CC:     Gao Feng <fgao@...ai8.com>,
        "David S . Miller" <davem@...emloft.net>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 4.9 066/219] tcp: sysctl: Fix a race to avoid
 unexpected 0 window from space

From: Gao Feng <fgao@...ai8.com>

[ Upstream commit c48367427a39ea0b85c7cf018fe4256627abfd9e ]

Because sysctl_tcp_adv_win_scale could be changed any time, so there
is one race in tcp_win_from_space.
For example,
1.sysctl_tcp_adv_win_scale<=0 (sysctl_tcp_adv_win_scale is negative now)
2.space>>(-sysctl_tcp_adv_win_scale) (sysctl_tcp_adv_win_scale is postive now)

As a result, tcp_win_from_space returns 0. It is unexpected.

Certainly if the compiler put the sysctl_tcp_adv_win_scale into one
register firstly, then use the register directly, it would be ok.
But we could not depend on the compiler behavior.

Signed-off-by: Gao Feng <fgao@...ai8.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
---
 include/net/tcp.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index caf35e062639..18f029bcb8c7 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1265,9 +1265,11 @@ void tcp_select_initial_window(int __space, __u32 mss, __u32 *rcv_wnd,
 
 static inline int tcp_win_from_space(int space)
 {
-	return sysctl_tcp_adv_win_scale<=0 ?
-		(space>>(-sysctl_tcp_adv_win_scale)) :
-		space - (space>>sysctl_tcp_adv_win_scale);
+	int tcp_adv_win_scale = sysctl_tcp_adv_win_scale;
+
+	return tcp_adv_win_scale <= 0 ?
+		(space>>(-tcp_adv_win_scale)) :
+		space - (space>>tcp_adv_win_scale);
 }
 
 /* Note: caller must be prepared to deal with negative returns */
-- 
2.14.1