linux-kernel - [PATCH 4.9 48/90] cifs: Fix a race condition with cifs_echo

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20200518173500.990164419@linuxfoundation.org>
Date:   Mon, 18 May 2020 19:36:26 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Ronnie Sahlberg <lsahlber@...hat.com>,
        Pavel Shilovsky <pshilov@...rosoft.com>,
        Steve French <stfrench@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.9 48/90] cifs: Fix a race condition with cifs_echo_request

From: Ronnie Sahlberg <lsahlber@...hat.com>

[ Upstream commit f2caf901c1b7ce65f9e6aef4217e3241039db768 ]

There is a race condition with how we send (or supress and don't send)
smb echos that will cause the client to incorrectly think the
server is unresponsive and thus needs to be reconnected.

Summary of the race condition:
 1) Daisy chaining scheduling creates a gap.
 2) If traffic comes unfortunate shortly after
    the last echo, the planned echo is suppressed.
 3) Due to the gap, the next echo transmission is delayed
    until after the timeout, which is set hard to twice
    the echo interval.

This is fixed by changing the timeouts from 2 to three times the echo interval.

Detailed description of the bug: https://lutz.donnerhacke.de/eng/Blog/Groundhog-Day-with-SMB-remount

Signed-off-by: Ronnie Sahlberg <lsahlber@...hat.com>
Reviewed-by: Pavel Shilovsky <pshilov@...rosoft.com>
Signed-off-by: Steve French <stfrench@...rosoft.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 fs/cifs/connect.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
index 37c8cac86431f..3545b237187a8 100644
--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -551,10 +551,10 @@ static bool
 server_unresponsive(struct TCP_Server_Info *server)
 {
 	/*
-	 * We need to wait 2 echo intervals to make sure we handle such
+	 * We need to wait 3 echo intervals to make sure we handle such
 	 * situations right:
 	 * 1s  client sends a normal SMB request
-	 * 2s  client gets a response
+	 * 3s  client gets a response
 	 * 30s echo workqueue job pops, and decides we got a response recently
 	 *     and don't need to send another
 	 * ...
@@ -563,9 +563,9 @@ server_unresponsive(struct TCP_Server_Info *server)
 	 */
 	if ((server->tcpStatus == CifsGood ||
 	    server->tcpStatus == CifsNeedNegotiate) &&
-	    time_after(jiffies, server->lstrp + 2 * server->echo_interval)) {
+	    time_after(jiffies, server->lstrp + 3 * server->echo_interval)) {
 		cifs_dbg(VFS, "Server %s has not responded in %lu seconds. Reconnecting...\n",
-			 server->hostname, (2 * server->echo_interval) / HZ);
+			 server->hostname, (3 * server->echo_interval) / HZ);
 		cifs_reconnect(server);
 		wake_up(&server->response_q);
 		return true;
-- 
2.20.1