fscrypt: fix dereference of NULL user_key_payload

From: Eric Biggers <[email protected]>

commit d60b5b7854c3d135b869f74fb93eaf63cbb1991a upstream. Please apply to 4.9-stable.

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.

---
 fs/crypto/keyinfo.c | 5 +++++
 1 file changed, 5 insertions(+)

index bb4606368eb1..a755fa1a0017 100644
@@ -108,6 +108,11 @@ 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+		/* key was revoked before we acquired its semaphore */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;

2.15.0.rc0.271.g36b669edcc-goog

Thanks for the backport, now queued up.

greg k-h