fscrypt: fix dereference of NULL user_key_payload

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: 88bd6ccdcdd6 ("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>

diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c
index 66e0728..169fefb 100644 (file)
--- a/fs/crypto/keyinfo.c
+++ b/fs/crypto/keyinfo.c
@@ -109,6 +109,11 @@ static int validate_user_key(struct fscrypt_info *crypt_info,
                goto out;
        }
        ukp = user_key_payload(keyring_key);
+       if (!ukp) {
+               /* key was revoked before we acquired its semaphore */
+               res = -EKEYREVOKED;
+               goto out;
+       }
        if (ukp->datalen != sizeof(struct fscrypt_key)) {
                res = -EINVAL;
                goto out;