Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
CC:     Gao Feng <fgao@ikuai8.com>,
        "David S . Miller" <davem@davemloft.net>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.4 033/115] tcp: sysctl: Fix a race to avoid
 unexpected 0 window from space
Thread-Topic: [PATCH AUTOSEL for 4.4 033/115] tcp: sysctl: Fix a race to avoid
 unexpected 0 window from space
Thread-Index: AQHTsz9Sg+6vIWnvyk2UbPEvPhCLLQ==
Date:   Sat, 3 Mar 2018 22:31:07 +0000
Message-ID: <20180303223010.27106-33-alexander.levin@microsoft.com>
References: <20180303223010.27106-1-alexander.levin@microsoft.com>
In-Reply-To: <20180303223010.27106-1-alexander.levin@microsoft.com>
Accept-Language: en-US
Content-Language: en-US
received-spf: None (protection.outlook.com: microsoft.com does not designate
 permitted sender hosts)
spamdiagnosticoutput: 1:99
spamdiagnosticmetadata: NSPM
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
X-MS-Exchange-CrossTenant-Network-Message-Id: d010e4a5-8588-42ed-8985-08d581578462
X-MS-Exchange-CrossTenant-originalarrivaltime: 03 Mar 2018 22:31:07.9946
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Hosted
X-MS-Exchange-CrossTenant-id: 72f988bf-86f1-41af-91ab-2d7cd011db47
X-MS-Exchange-Transport-CrossTenantHeadersStamped: MW2PR2101MB1067
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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<=3D0 (sysctl_tcp_adv_win_scale is negative now)
2.space>>(-sysctl_tcp_adv_win_scale) (sysctl_tcp_adv_win_scale is postive n=
ow)

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 cecb0e0eff06..a3696b778757 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1199,9 +1199,11 @@ void tcp_select_initial_window(int __space, __u32 ms=
s, __u32 *rcv_wnd,
=20
 static inline int tcp_win_from_space(int space)
 {
-	return sysctl_tcp_adv_win_scale<=3D0 ?
-		(space>>(-sysctl_tcp_adv_win_scale)) :
-		space - (space>>sysctl_tcp_adv_win_scale);
+	int tcp_adv_win_scale =3D sysctl_tcp_adv_win_scale;
+
+	return tcp_adv_win_scale <=3D 0 ?
+		(space>>(-tcp_adv_win_scale)) :
+		space - (space>>tcp_adv_win_scale);
 }
=20
 /* Note: caller must be prepared to deal with negative returns */
--=20
2.14.1