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Subject[PATCH 4.9 052/101] X.509: unpack RSA signatureValue field from BIT STRING
4.9-stable review patch.  If anyone has any objections, please let me know.


From: Maciej S. Szmigiero <>

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 <>
Fixes: c26fd69fa009 ("X.509: Add a crypto key parser for binary (DER) X.509 certificates")
Signed-off-by: James Morris <>
Signed-off-by: Greg Kroah-Hartman <>

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

--- 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, s
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;

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