CIFS: avoid using MID 0xFFFF

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>
CC: Stable <stable@vger.kernel.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/cifs/smb1ops.c b/fs/cifs/smb1ops.c
index b7421a096319..514810694c0f 100644
--- a/fs/cifs/smb1ops.c
+++ b/fs/cifs/smb1ops.c
@@ -171,6 +171,9 @@ cifs_get_next_mid(struct TCP_Server_Info *server)
         /* 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.