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

Sasha Levin
2018-03-03T23:56:16Z

From: Gao Feng <fgao@ikuai8.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@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.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