linux-kernel - [PATCH v2] X.509: unpack RSA signatureValue field from BIT STRING

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Date:   Tue, 17 Apr 2018 15:39:33 +0200
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     David Howells <dhowells@...hat.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Tom Lendacky <thomas.lendacky@....com>,
        Gary Hook <gary.hook@....com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2] X.509: unpack RSA signatureValue field from BIT STRING

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@...iej.szmigiero.name>
Fixes: c26fd69fa009 ("X.509: Add a crypto key parser for binary (DER) X.509 certificates")
Cc: stable@...r.kernel.org
---
This is a resend of a patch that was previously submitted in one series
with CCP driver changes since this particular patch should go through
the security (rather than crypto) tree.

Changes from v1: Change '!' to '== 0'.

 crypto/asymmetric_keys/x509_cert_parser.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/crypto/asymmetric_keys/x509_cert_parser.c b/crypto/asymmetric_keys/x509_cert_parser.c
index 7d81e6bb461a..b6cabac4b62b 100644
--- a/crypto/asymmetric_keys/x509_cert_parser.c
+++ b/crypto/asymmetric_keys/x509_cert_parser.c
@@ -249,6 +249,15 @@ int x509_note_signature(void *context, size_t hdrlen,
 		return -EINVAL;
 	}
 
+	if (strcmp(ctx->cert->sig->pkey_algo, "rsa") == 0) {
+		/* 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;