SMB: Validate negotiate (to protect against downgrade) even if signing off

As long as signing is supported (ie not a guest user connection) and
connection is SMB3 or SMB3.02, then validate negotiate (protect
against man in the middle downgrade attacks).  We had been doing this
only when signing was required, not when signing was just enabled,
but this more closely matches recommended SMB3 behavior and is
better security.  Suggested by Metze.

Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Acked-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
CC: Stable <stable@vger.kernel.org>

fs/cifs/smb2pdu.c

@@ -656,15 +656,22 @@ int smb3_validate_negotiate(const unsigned int xid, struct cifs_tcon *tcon)
 
 	/*
 	 * validation ioctl must be signed, so no point sending this if we
-	 * can not sign it.  We could eventually change this to selectively
+	 * can not sign it (ie are not known user).  Even if signing is not
+	 * required (enabled but not negotiated), in those cases we selectively
 	 * sign just this, the first and only signed request on a connection.
-	 * This is good enough for now since a user who wants better security
-	 * would also enable signing on the mount. Having validation of
-	 * negotiate info for signed connections helps reduce attack vectors
+	 * Having validation of negotiate info  helps reduce attack vectors.
 	 */
-	if (tcon->ses->server->sign == false)
+	if (tcon->ses->session_flags & SMB2_SESSION_FLAG_IS_GUEST)
 		return 0; /* validation requires signing */
 
+	if (tcon->ses->user_name == NULL) {
+		cifs_dbg(FYI, "Can't validate negotiate: null user mount\n");
+		return 0; /* validation requires signing */
+	}
+
+	if (tcon->ses->session_flags & SMB2_SESSION_FLAG_IS_NULL)
+		cifs_dbg(VFS, "Unexpected null user (anonymous) auth flag sent by server\n");
+
 	vneg_inbuf.Capabilities =
 			cpu_to_le32(tcon->ses->server->vals->req_capabilities);
 	memcpy(vneg_inbuf.Guid, tcon->ses->server->client_guid,