NFS: Fix NFSv2 security settings

For a while now any NFSv2 mount where sec= is specified uses
AUTH_NULL. If sec= is not specified, the mount uses AUTH_UNIX.
Commit e68fd7c8071d ("mount: use sec= that was specified on the
command line") attempted to address a very similar problem with
NFSv3, and should have fixed this too, but it has a bug.

The MNTv1 MNT procedure does not return a list of security flavors,
so our client makes up a list containing just AUTH_NULL. This should
enable nfs_verify_authflavors() to assign the sec= specified flavor,
but instead, it incorrectly sets it to AUTH_NULL.

I expect this would also be a problem for any NFSv3 server whose
MNTv3 MNT procedure returned a security flavor list containing only
AUTH_NULL.

Fixes: e68fd7c8071d ("mount: use sec= that was specified on ... ")
BugLink: https://bugzilla.linux-nfs.org/show_bug.cgi?id=310
Signed-off-by: Chuck Lever <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/super.c

···
 			rpc_authflavor_t *server_authlist, unsigned int count)
 {
 	rpc_authflavor_t flavor = RPC_AUTH_MAXFLAVOR;
+	bool found_auth_null = false;
 	unsigned int i;
-	int use_auth_null = false;
 
 	/*
 	 * If the sec= mount option is used, the specified flavor or AUTH_NULL
···
 	 * AUTH_NULL has a special meaning when it's in the server list - it
 	 * means that the server will ignore the rpc creds, so any flavor
 	 * can be used but still use the sec= that was specified.
+	 *
+	 * Note also that the MNT procedure in MNTv1 does not return a list
+	 * of supported security flavors. In this case, nfs_mount() fabricates
+	 * a security flavor list containing just AUTH_NULL.
 	 */
 	for (i = 0; i < count; i++) {
 		flavor = server_authlist[i];
···
 			goto out;
 
 		if (flavor == RPC_AUTH_NULL)
-			use_auth_null = true;
+			found_auth_null = true;
 	}
 
-	if (use_auth_null) {
-		flavor = RPC_AUTH_NULL;
+	if (found_auth_null) {
+		flavor = args->auth_info.flavors[0];
 		goto out;
 	}