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

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Message-ID: <88da0c24-385c-64a9-6d56-36696d433bc7@maciej.szmigiero.name>
Date:   Tue, 6 Mar 2018 20:13:18 +0100
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>, keyrings@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] 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 without changes 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.

 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 ce2df8c9c583..88c26a4538ae 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")) {
+		/* 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;