[PATCH 4.19 74/93] CIFS: avoid using MID 0xFFFF

From: Greg Kroah-Hartman
Date: Sun Oct 27 2019 - 17:32:01 EST

From: Roberto Bergantinos Corpas <rbergant@xxxxxxxxxx>

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@xxxxxxxxxx>
Reviewed-by: Ronnie Sahlberg <lsahlber@xxxxxxxxxx>
Reviewed-by: Aurelien Aptel <aaptel@xxxxxxxx>
Signed-off-by: Steve French <stfrench@xxxxxxxxxxxxx>
CC: Stable <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

fs/cifs/smb1ops.c | 3 +++
1 file changed, 3 insertions(+)

--- a/fs/cifs/smb1ops.c
+++ b/fs/cifs/smb1ops.c
@@ -183,6 +183,9 @@ cifs_get_next_mid(struct TCP_Server_Info
/* we do not want to loop forever */
last_mid = cur_mid;
+ /* avoid 0xFFFF MID */
+ if (cur_mid == 0xffff)
+ cur_mid++;

* This nested loop looks more expensive than it is.