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 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

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-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From foo@baz Sun Mar 18 16:55:33 CET 2018

From: Gao Feng fgao@ikuai8.com 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 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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/net/tcp.h | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -1265,9 +1265,11 @@ 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 fgao@ikuai8.com are

queue-4.9/tcp-sysctl-fix-a-race-to-avoid-unexpected-0-window-from-space.patch