X.509: unpack RSA signatureValue field from BIT STRING

commit b65c32ec5a942ab3ada93a048089a938918aba7f upstream.

The signatureValue field of a X.509 certificate is encoded as a BIT STRING.
For RSA signatures this BIT STRING is of so-called primitive subtype, which
contains a u8 prefix indicating a count of unused bits in the encoding.

We have to strip this prefix from signature data, just as we already do for
key data in x509_extract_key_data() function.

This wasn't noticed earlier because this prefix byte is zero for RSA key
sizes divisible by 8. Since BIT STRING is a big-endian encoding adding zero
prefixes has no bearing on its value.

The signature length, however was incorrect, which is a problem for RSA
implementations that need it to be exactly correct (like AMD CCP).

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Fixes: c26fd69fa009 ("X.509: Add a crypto key parser for binary (DER) X.509 certificates")
Signed-off-by: James Morris <james.morris@microsoft.com>
[bwh: Backported to 3.16:
 - x509_certificate::sig is a structure, not a pointer
 - public_key_signature::pkey_algo is an enumeration type, not a string]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

1 files changed, 9 insertions, 0 deletions

diff --git a/crypto/asymmetric_keys/x509_cert_parser.c b/crypto/asymmetric_keys/x509_cert_parser.c
index 3d74e4fac6bb..641f63899f92 100644
--- a/crypto/asymmetric_keys/x509_cert_parser.c
+++ b/crypto/asymmetric_keys/x509_cert_parser.c
@@ -205,6 +205,15 @@ int x509_note_signature(void *context, size_t hdrlen,
 		return -EINVAL;
 	}
 
+	if (ctx->cert->sig.pkey_algo == PKEY_ALGO_RSA) {
+		/* Discard the BIT STRING metadata */
+		if (vlen < 1 || *(const u8 *)value != 0)
+			return -EBADMSG;
+
+		value++;
+		vlen--;
+	}
+
 	ctx->cert->raw_sig = value;
 	ctx->cert->raw_sig_size = vlen;
 	return 0;