[PATCH 3.16 082/136] CIFS: avoid using MID 0xFFFF

Ben Hutchings
2019-12-17T00:56:13Z

From: Roberto Bergantinos Corpas <rbergant@redhat.com>

commit 03d9a9fe3f3aec508e485dd3dcfa1e99933b4bdb upstream.

According to MS-CIFS specification MID 0xFFFF should not be used by the
CIFS client, but we actually do. Besides, this has proven to cause races
leading to oops between SendReceive2/cifs_demultiplex_thread. On SMB1,
MID is a 2 byte value easy to reach in CurrentMid which may conflict with
an oplock break notification request coming from server

Signed-off-by: Roberto Bergantinos Corpas <rbergant@redhat.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/cifs/smb1ops.c | 3 +++
1 file changed, 3 insertions(+)

--- a/fs/cifs/smb1ops.c
+++ b/fs/cifs/smb1ops.c
@@ -180,6 +180,9 @@ cifs_get_next_mid(struct TCP_Server_Info
/* we do not want to loop forever */
last_mid = cur_mid;
cur_mid++;
+ /* avoid 0xFFFF MID */
+ if (cur_mid == 0xffff)
+ cur_mid++;

/*
* This nested loop looks more expensive than it is.