[PATCH 2/2] SMB3: Fix SMB3.1.1 guest mounts to Samba

[Tyler Hicks]

From: Steve French <stfrench at microsoft.com>

BugLink: https://launchpad.net/bugs/1821053

Workaround problem with Samba responses to SMB3.1.1
null user (guest) mounts.  The server doesn't set the
expected flag in the session setup response so we have
to do a similar check to what is done in smb3_validate_negotiate
where we also check if the user is a null user (but not sec=krb5
since username might not be passed in on mount for Kerberos case).

Note that the commit below tightened the conditions and forced signing
for the SMB2-TreeConnect commands as per MS-SMB2.
However, this should only apply to normal user sessions and not for
cases where there is no user (even if server forgets to set the flag
in the response) since we don't have anything useful to sign with.
This is especially important now that the more secure SMB3.1.1 protocol
is in the default dialect list.

An earlier patch ("cifs: allow guest mounts to work for smb3.11") fixed
the guest mounts to Windows.

    Fixes: 6188f28bf608 ("Tree connect for SMB3.1.1 must be signed for non-encrypted shares")

Reviewed-by: Ronnie Sahlberg <lsahlber at redhat.com>
Reviewed-by: Paulo Alcantara <palcantara at suse.de>
CC: Stable <stable at vger.kernel.org>
Signed-off-by: Steve French <stfrench at microsoft.com>
(cherry picked from commit 8c11a607d1d9cd6e7f01fd6b03923597fb0ef95a)
Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
---
 fs/cifs/smb2pdu.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/cifs/smb2pdu.c b/fs/cifs/smb2pdu.c
index a68bed9c74a9..104905732fbe 100644
--- a/fs/cifs/smb2pdu.c
+++ b/fs/cifs/smb2pdu.c
@@ -1608,10 +1608,13 @@ SMB2_tcon(const unsigned int xid, struct cifs_ses *ses, const char *tree,
 	/*
 	 * 3.11 tcon req must be signed if not encrypted. See MS-SMB2 3.2.4.1.1
 	 * unless it is guest or anonymous user. See MS-SMB2 3.2.5.3.1
+	 * (Samba servers don't always set the flag so also check if null user)
 	 */
 	if ((ses->server->dialect == SMB311_PROT_ID) &&
 	    !smb3_encryption_required(tcon) &&
-	    !(ses->session_flags & (SMB2_SESSION_FLAG_IS_GUEST|SMB2_SESSION_FLAG_IS_NULL)))
+	    !(ses->session_flags &
+		    (SMB2_SESSION_FLAG_IS_GUEST|SMB2_SESSION_FLAG_IS_NULL)) &&
+	    ((ses->user_name != NULL) || (ses->sectype == Kerberos)))
 		req->sync_hdr.Flags |= SMB2_FLAGS_SIGNED;
 
 	memset(&rqst, 0, sizeof(struct smb_rqst));
-- 
2.7.4