fscrypt: fix dereference of NULL user_key_payload

commit d60b5b78 upstream.

When an fscrypt-encrypted file is opened, we request the file's master
key from the keyrings service as a logon key, then access its payload.
However, a revoked key has a NULL payload, and we failed to check for
this.  request_key() *does* skip revoked keys, but there is still a
window where the key can be revoked before we acquire its semaphore.

Fix it by checking for a NULL payload, treating it like a key which was
already revoked at the time it was requested.

Fixes: 88bd6ccd ("ext4 crypto: add encryption key management facilities")
Reviewed-by: James Morris <james.l.morris@oracle.com>
Cc: <stable@vger.kernel.org>    [v4.1+]
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/ext4/crypto_key.c

@@ -204,6 +204,12 @@ int ext4_get_encryption_info(struct inode *inode)
 	}
 	down_read(&keyring_key->sem);
 	ukp = user_key_payload(keyring_key);
+	if (!ukp) {
+		/* key was revoked before we acquired its semaphore */
+		res = -EKEYREVOKED;
+		up_read(&keyring_key->sem);
+		goto out;
+	}
 	if (ukp->datalen != sizeof(struct ext4_encryption_key)) {
 		res = -EINVAL;
 		up_read(&keyring_key->sem);

fs/f2fs/crypto_key.c

@@ -195,6 +195,12 @@ int f2fs_get_encryption_info(struct inode *inode)
 	}
 	down_read(&keyring_key->sem);
 	ukp = user_key_payload(keyring_key);
+	if (!ukp) {
+		/* key was revoked before we acquired its semaphore */
+		res = -EKEYREVOKED;
+		up_read(&keyring_key->sem);
+		goto out;
+	}
 	if (ukp->datalen != sizeof(struct f2fs_encryption_key)) {
 		res = -EINVAL;
 		up_read(&keyring_key->sem);