[PATCH AUTOSEL 4.19 17/47] cifs: Fix a race condition with cifs_echo_request

Sasha Levin
2019-07-26T13:54:56Z

From: Ronnie Sahlberg <lsahlber@redhat.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@redhat.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.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 f31339db45fd..c53a2e86ed54 100644
--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -563,10 +563,10 @@ 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
* ...
@@ -575,9 +575,9 @@ 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