linux-kernel - Re: [PATCH] X.509: Fix determination of self-signedness

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Date:	Fri, 18 Dec 2015 09:42:52 -0500
From:	Josh Boyer <jwboyer@...oraproject.org>
To:	David Howells <dhowells@...hat.com>
Cc:	keyrings@...r.kernel.org,
	linux-security-module <linux-security-module@...r.kernel.org>,
	David Woodhouse <David.Woodhouse@...el.com>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] X.509: Fix determination of self-signedness

On Thu, Dec 17, 2015 at 7:03 PM, David Howells <dhowells@...hat.com> wrote:
> Fix determination of whether an X.509 certificate is self-signed or not.
>
> It is currently assumed that a cert is self-signed if has no
> authorityKeyIdentifier or the authorityKeyIdentifier matches the
> subjectKeyIdentifier.  However, it is possible to encounter a certificate
> that has neither AKID not SKID but is not self-signed.
>
> This symptoms of this show up as an attempt to load a certificate failing
> with -ERANGE or -EBADMSG, produced from the RSA module when the result of
> calculating "m = s^e mod n" is checked.
>
> To fix this, don't check to see if a certificate is self-signed if the
> Issuer and Subject names differ.
>
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: David Woodhouse <David.Woodhouse@...el.com>

Should this also be Cc'd to stable?

josh

> ---
>
>  crypto/asymmetric_keys/x509_public_key.c |   11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/crypto/asymmetric_keys/x509_public_key.c b/crypto/asymmetric_keys/x509_public_key.c
> index 2a44b3752471..6236e7996f19 100644
> --- a/crypto/asymmetric_keys/x509_public_key.c
> +++ b/crypto/asymmetric_keys/x509_public_key.c
> @@ -313,9 +313,14 @@ static int x509_key_preparse(struct key_preparsed_payload *prep)
>         cert->pub->id_type = PKEY_ID_X509;
>
>         /* Check the signature on the key if it appears to be self-signed */
> -       if ((!cert->akid_skid && !cert->akid_id) ||
> -           asymmetric_key_id_same(cert->skid, cert->akid_skid) ||
> -           asymmetric_key_id_same(cert->id, cert->akid_id)) {
> +       if ((!cert->akid_skid && !cert->akid_id)) {
> +               if (cert->raw_issuer_size == cert->raw_subject_size &&
> +                   memcmp(cert->raw_issuer, cert->raw_subject,
> +                          cert->raw_subject_size) == 0)
> +                       goto self_signed;
> +       } else if (asymmetric_key_id_same(cert->skid, cert->akid_skid) ||
> +                  asymmetric_key_id_same(cert->id, cert->akid_id)) {
> +self_signed:
>                 ret = x509_check_signature(cert->pub, cert); /* self-signed */
>                 if (ret < 0)
>                         goto error_free_cert;
>
--
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