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

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Message-ID: <20151218000302.29802.28710.stgit@warthog.procyon.org.uk>
Date:	Fri, 18 Dec 2015 00:03:02 +0000
From:	David Howells <dhowells@...hat.com>
To:	keyrings@...r.kernel.org
Cc:	dhowells@...hat.com, linux-security-module@...r.kernel.org,
	David Woodhouse <David.Woodhouse@...el.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] X.509: Fix determination of self-signedness

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>
---

 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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