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From: "Maciej S. Szmigiero" <>
To: David Howells <>
Cc: Herbert Xu <>,
	"David S. Miller" <>,
	Tom Lendacky <>,
	Gary Hook <>, James Morris <>,
	"Serge E. Hallyn" <>,,,,
Subject: [PATCH v2] X.509: unpack RSA signatureValue field from BIT STRING
Date: Tue, 17 Apr 2018 15:39:33 +0200	[thread overview]
Message-ID: <> (raw)

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 <>
Fixes: c26fd69fa009 ("X.509: Add a crypto key parser for binary (DER) X.509 certificates")
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;

             reply	other threads:[~2018-04-17 13:39 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
2018-04-17 13:39 ` Maciej S. Szmigiero [this message]
2018-04-17 15:07   ` Kamil Konieczny
2018-04-17 15:11     ` Maciej S. Szmigiero

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