cifs: check for bytes_remaining going to zero in CIFS_SessSetup

It's possible that when we go to decode the string area in the
SESSION_SETUP response, that bytes_remaining will be 0. Decrementing it at
that point will mean that it can go "negative" and wrap. Check for a
bytes_remaining value of 0, and don't try to decode the string area if
that's the case.

Cc: stable@kernel.org
Reported-and-Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/fs/cifs/sess.c b/fs/cifs/sess.c
index 2e2c91103529..645114ad0a10 100644
--- a/fs/cifs/sess.c
+++ b/fs/cifs/sess.c
@@ -916,7 +916,9 @@ ssetup_ntlmssp_authenticate:
         }
 
         /* BB check if Unicode and decode strings */
-        if (smb_buf->Flags2 & SMBFLG2_UNICODE) {
+        if (bytes_remaining == 0) {
+                /* no string area to decode, do nothing */
+        } else if (smb_buf->Flags2 & SMBFLG2_UNICODE) {
                 /* unicode string area must be word-aligned */
                 if (((unsigned long) bcc_ptr - (unsigned long) smb_buf) % 2) {
                         ++bcc_ptr;