NFS: reject request for id_legacy key without auxdata

nfs_idmap_legacy_upcall() is supposed to be called with 'aux' pointing
to a 'struct idmap', via the call to request_key_with_auxdata() in
nfs_idmap_request_key().

However it can also be reached via the request_key() system call in
which case 'aux' will be NULL, causing a NULL pointer dereference in
nfs_idmap_prepare_pipe_upcall(), assuming that the key description is
valid enough to get that far.

Fix this by making nfs_idmap_legacy_upcall() negate the key if no
auxdata is provided.

As usual, this bug was found by syzkaller.  A simple reproducer using
the command-line keyctl program is:

    keyctl request2 id_legacy uid:0 '' @s

Fixes: 57e62324 ("NFS: Store the legacy idmapper result in the keyring")
Reported-by: syzbot+5dfdbcf7b3eb5912abbb@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org> # v3.4+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Trond Myklebust <trondmy@gmail.com>

fs/nfs/nfs4idmap.c

@@ -568,9 +568,13 @@ static int nfs_idmap_legacy_upcall(struct key_construction *cons,
 	struct idmap_msg *im;
 	struct idmap *idmap = (struct idmap *)aux;
 	struct key *key = cons->key;
-	int ret = -ENOMEM;
+	int ret = -ENOKEY;
+
+	if (!aux)
+		goto out1;
 
 	/* msg and im are freed in idmap_pipe_destroy_msg */
+	ret = -ENOMEM;
 	data = kzalloc(sizeof(*data), GFP_KERNEL);
 	if (!data)
 		goto out1;