Linux Kernels

X.509: unpack RSA signatureValue field from BIT STRING

This change “X.509: unpack RSA signatureValue field from BIT STRING” (commit b65c32e) in Linux kernel is authored by Maciej S. Szmigiero <mail [at] maciej.szmigiero.name> on Sat May 19 14:23:54 2018 +0200.

Description of "X.509: unpack RSA signatureValue field from BIT STRING"

The change “X.509: unpack RSA signatureValue field from BIT STRING” introduces changes as follows.

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@maciej.szmigiero.name>
Fixes: c26fd69fa009 ("X.509: Add a crypto key parser for binary (DER) X.509 certificates")
Cc: stable@vger.kernel.org
Signed-off-by: James Morris <james.morris@microsoft.com>

Linux kernel releases containing commit b65c32e

The Linux kernel releases containing this commit are as follows.

Linux kernel code changes in "X.509: unpack RSA signatureValue field from BIT STRING"

There are 9 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 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;

The commit for this change in Linux stable tree is b65c32e (patch).

Last modified: 2020/01/11 06:43