[PATCH 4.9] X.509: fix NULL dereference when restricting key with unsupported_sig

[Eric Biggers <ebiggers3@gmail.com>]

From: Eric Biggers <ebiggers@google.com>

commit 4b34968e77ad09628cfb3c4a7daf2adc2cefc6e8 upstream.
[Please apply to 4.9-stable.  I dropped the portion of the patch
pertaining to KEYCTL_RESTRICT_KEYRING which was added in v4.12.]

The asymmetric key type allows an X.509 certificate to be added even if
its signature's hash algorithm is not available in the crypto API.  In
that case 'payload.data[asym_auth]' will be NULL.  But the key
restriction code failed to check for this case before trying to use the
signature, resulting in a NULL pointer dereference in
key_or_keyring_common() or in restrict_link_by_signature().

Fix this by returning -ENOPKG when the signature is unsupported.

Reproducer when all the CONFIG_CRYPTO_SHA512* options are disabled and
keyctl has support for the 'restrict_keyring' command:

    keyctl new_session
    keyctl restrict_keyring @s asymmetric builtin_trusted
    openssl req -new -sha512 -x509 -batch -nodes -outform der \
        | keyctl padd asymmetric desc @s

Fixes: a511e1af8b12 ("KEYS: Move the point of trust determination to __key_link()")
Cc: <stable@vger.kernel.org> # v4.7+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
---
 crypto/asymmetric_keys/restrict.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/crypto/asymmetric_keys/restrict.c b/crypto/asymmetric_keys/restrict.c
index 19d1afb9890f..09b1374dc619 100644
--- a/crypto/asymmetric_keys/restrict.c
+++ b/crypto/asymmetric_keys/restrict.c
@@ -66,8 +66,9 @@ __setup("ca_keys=", ca_keys_setup);
  *
  * Returns 0 if the new certificate was accepted, -ENOKEY if we couldn't find a
  * matching parent certificate in the trusted list, -EKEYREJECTED if the
- * signature check fails or the key is blacklisted and some other error if
- * there is a matching certificate but the signature check cannot be performed.
+ * signature check fails or the key is blacklisted, -ENOPKG if the signature
+ * uses unsupported crypto, or some other error if there is a matching
+ * certificate but the signature check cannot be performed.
  */
 int restrict_link_by_signature(struct key *trust_keyring,
 			       const struct key_type *type,
@@ -86,6 +87,8 @@ int restrict_link_by_signature(struct key *trust_keyring,
 		return -EOPNOTSUPP;
 
 	sig = payload->data[asym_auth];
+	if (!sig)
+		return -ENOPKG;
 	if (!sig->auth_ids[0] && !sig->auth_ids[1])
 		return -ENOKEY;
 
-- 
2.16.1.291.g4437f3f132-goog