SMB: fix validate negotiate info uninitialised memory use

An undersize validate negotiate info server response causes the client
to use uninitialised memory for struct validate_negotiate_info_rsp
comparisons of Dialect, SecurityMode and/or Capabilities members.

Link: https://bugzilla.samba.org/show_bug.cgi?id=13092
Fixes: 7db0a6ef ("SMB3: Work around mount failure when using SMB3 dialect to Macs")
Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/smb2pdu.c

@@ -727,7 +727,8 @@ int smb3_validate_negotiate(const unsigned int xid, struct cifs_tcon *tcon)
 			 rsplen);
 
 		/* relax check since Mac returns max bufsize allowed on ioctl */
-		if (rsplen > CIFSMaxBufSize)
+		if ((rsplen > CIFSMaxBufSize)
+		     || (rsplen < sizeof(struct validate_negotiate_info_rsp)))
 			goto err_rsp_free;
 	}