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    Subject[PATCH 4.14 089/157] X.509: unpack RSA signatureValue field from BIT STRING
    Date
    4.14-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Maciej S. Szmigiero <mail@maciej.szmigiero.name>

    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 <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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    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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