Patch "tcp: sysctl: Fix a race to avoid unexpected 0 window from space" has been added to the 3.18-stable tree

This is a note to let you know that I've just added the patch titled

tcp: sysctl: Fix a race to avoid unexpected 0 window from space

to the 3.18-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tcp-sysctl-fix-a-race-to-avoid-unexpected-0-window-from-space.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Gao Feng <[email protected]>
Date: Fri, 24 Mar 2017 07:05:12 +0800
Subject: tcp: sysctl: Fix a race to avoid unexpected 0 window from space

From: Gao Feng <[email protected]>

[ 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.

---
 include/net/tcp.h |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

@@ -1099,9 +1099,11 @@ void tcp_select_initial_window(int __spa
 
 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 */ 

Patches currently in stable-queue which might be from [email protected] are

queue-3.18/tcp-sysctl-fix-a-race-to-avoid-unexpected-0-window-from-space.patch