X.509: fix BUG_ON() when hash algorithm is unsupported

The X.509 parser mishandles the case where the certificate's signature's
hash algorithm is not available in the crypto API.  In this case,
x509_get_sig_params() doesn't allocate the cert->sig->digest buffer;
this part seems to be intentional.  However,
public_key_verify_signature() is still called via
x509_check_for_self_signed(), which triggers the 'BUG_ON(!sig->digest)'.

Fix this by making public_key_verify_signature() return -ENOPKG if the
hash buffer has not been allocated.

Reproducer when all the CONFIG_CRYPTO_SHA512* options are disabled:

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

Fixes: 6c2dc5ae ("X.509: Extract signature digest and make self-signed cert checks earlier")
Reported-by: Paolo Valente <paolo.valente@linaro.org>
Cc: Paolo Valente <paolo.valente@linaro.org>
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/public_key.c

@@ -79,9 +79,11 @@ int public_key_verify_signature(const struct public_key *pkey,
 
 	BUG_ON(!pkey);
 	BUG_ON(!sig);
-	BUG_ON(!sig->digest);
 	BUG_ON(!sig->s);
 
+	if (!sig->digest)
+		return -ENOPKG;
+
 	alg_name = sig->pkey_algo;
 	if (strcmp(sig->pkey_algo, "rsa") == 0) {
 		/* The data wangled by the RSA algorithm is typically padded