linux/crypto/asymmetric_keys/restrict.c
Andrew Zaborowski 7d30198ee2 keys: X.509 public key issuer lookup without AKID
There are non-root X.509 v3 certificates in use out there that contain
no Authority Key Identifier extension (RFC5280 section 4.2.1.1).  For
trust verification purposes the kernel asymmetric key type keeps two
struct asymmetric_key_id instances that the key can be looked up by,
and another two to look up the key's issuer.  The x509 public key type
and the PKCS7 type generate them from the SKID and AKID extensions in
the certificate.  In effect current code has no way to look up the
issuer certificate for verification without the AKID.

To remedy this, add a third asymmetric_key_id blob to the arrays in
both asymmetric_key_id's (for certficate subject) and in the
public_keys_signature's auth_ids (for issuer lookup), using just raw
subject and issuer DNs from the certificate.  Adapt
asymmetric_key_ids() and its callers to use the third ID for lookups
when none of the other two are available.  Attempt to keep the logic
intact when they are, to minimise behaviour changes.  Adapt the
restrict functions' NULL-checks to include that ID too.  Do not modify
the lookup logic in pkcs7_verify.c, the AKID extensions are still
required there.

Internally use a new "dn:" prefix to the search specifier string
generated for the key lookup in find_asymmetric_key().  This tells
asymmetric_key_match_preparse to only match the data against the raw
DN in the third ID and shouldn't conflict with search specifiers
already in use.

In effect implement what (2) in the struct asymmetric_key_id comment
(include/keys/asymmetric-type.h) is probably talking about already, so
do not modify that comment.  It is also how "openssl verify" looks up
issuer certificates without the AKID available.  Lookups by the raw
DN are unambiguous only provided that the CAs respect the condition in
RFC5280 4.2.1.1 that the AKID may only be omitted if the CA uses
a single signing key.

The following is an example of two things that this change enables.
A self-signed ceritficate is generated following the example from
https://letsencrypt.org/docs/certificates-for-localhost/, and can be
looked up by an identifier and verified against itself by linking to a
restricted keyring -- both things not possible before due to the missing
AKID extension:

$ openssl req -x509 -out localhost.crt -outform DER -keyout localhost.key \
  -newkey rsa:2048 -nodes -sha256 \
  -subj '/CN=localhost' -extensions EXT -config <( \
   echo -e "[dn]\nCN=localhost\n[req]\ndistinguished_name = dn\n[EXT]\n" \
          "subjectAltName=DNS:localhost\nkeyUsage=digitalSignature\n" \
	  "extendedKeyUsage=serverAuth")
$ keyring=`keyctl newring test @u`
$ trusted=`keyctl padd asymmetric trusted $keyring < localhost.crt`; \
  echo $trusted
39726322
$ keyctl search $keyring asymmetric dn:3112301006035504030c096c6f63616c686f7374
39726322
$ keyctl restrict_keyring $keyring asymmetric key_or_keyring:$trusted
$ keyctl padd asymmetric verified $keyring < localhost.crt

Signed-off-by: Andrew Zaborowski <andrew.zaborowski@intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
2022-01-09 00:18:42 +02:00

276 lines
8.3 KiB
C

// SPDX-License-Identifier: GPL-2.0-or-later
/* Instantiate a public key crypto key from an X.509 Certificate
*
* Copyright (C) 2012, 2016 Red Hat, Inc. All Rights Reserved.
* Written by David Howells (dhowells@redhat.com)
*/
#define pr_fmt(fmt) "ASYM: "fmt
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/err.h>
#include <crypto/public_key.h>
#include "asymmetric_keys.h"
static bool use_builtin_keys;
static struct asymmetric_key_id *ca_keyid;
#ifndef MODULE
static struct {
struct asymmetric_key_id id;
unsigned char data[10];
} cakey;
static int __init ca_keys_setup(char *str)
{
if (!str) /* default system keyring */
return 1;
if (strncmp(str, "id:", 3) == 0) {
struct asymmetric_key_id *p = &cakey.id;
size_t hexlen = (strlen(str) - 3) / 2;
int ret;
if (hexlen == 0 || hexlen > sizeof(cakey.data)) {
pr_err("Missing or invalid ca_keys id\n");
return 1;
}
ret = __asymmetric_key_hex_to_key_id(str + 3, p, hexlen);
if (ret < 0)
pr_err("Unparsable ca_keys id hex string\n");
else
ca_keyid = p; /* owner key 'id:xxxxxx' */
} else if (strcmp(str, "builtin") == 0) {
use_builtin_keys = true;
}
return 1;
}
__setup("ca_keys=", ca_keys_setup);
#endif
/**
* restrict_link_by_signature - Restrict additions to a ring of public keys
* @dest_keyring: Keyring being linked to.
* @type: The type of key being added.
* @payload: The payload of the new key.
* @trust_keyring: A ring of keys that can be used to vouch for the new cert.
*
* Check the new certificate against the ones in the trust keyring. If one of
* those is the signing key and validates the new certificate, then mark the
* new certificate as being trusted.
*
* Returns 0 if the new certificate was accepted, -ENOKEY if we couldn't find a
* matching parent certificate in the trusted list, -EKEYREJECTED if the
* signature check fails or the key is blacklisted, -ENOPKG if the signature
* uses unsupported crypto, or some other error if there is a matching
* certificate but the signature check cannot be performed.
*/
int restrict_link_by_signature(struct key *dest_keyring,
const struct key_type *type,
const union key_payload *payload,
struct key *trust_keyring)
{
const struct public_key_signature *sig;
struct key *key;
int ret;
pr_devel("==>%s()\n", __func__);
if (!trust_keyring)
return -ENOKEY;
if (type != &key_type_asymmetric)
return -EOPNOTSUPP;
sig = payload->data[asym_auth];
if (!sig)
return -ENOPKG;
if (!sig->auth_ids[0] && !sig->auth_ids[1] && !sig->auth_ids[2])
return -ENOKEY;
if (ca_keyid && !asymmetric_key_id_partial(sig->auth_ids[1], ca_keyid))
return -EPERM;
/* See if we have a key that signed this one. */
key = find_asymmetric_key(trust_keyring,
sig->auth_ids[0], sig->auth_ids[1],
sig->auth_ids[2], false);
if (IS_ERR(key))
return -ENOKEY;
if (use_builtin_keys && !test_bit(KEY_FLAG_BUILTIN, &key->flags))
ret = -ENOKEY;
else
ret = verify_signature(key, sig);
key_put(key);
return ret;
}
static bool match_either_id(const struct asymmetric_key_id **pair,
const struct asymmetric_key_id *single)
{
return (asymmetric_key_id_same(pair[0], single) ||
asymmetric_key_id_same(pair[1], single));
}
static int key_or_keyring_common(struct key *dest_keyring,
const struct key_type *type,
const union key_payload *payload,
struct key *trusted, bool check_dest)
{
const struct public_key_signature *sig;
struct key *key = NULL;
int ret;
pr_devel("==>%s()\n", __func__);
if (!dest_keyring)
return -ENOKEY;
else if (dest_keyring->type != &key_type_keyring)
return -EOPNOTSUPP;
if (!trusted && !check_dest)
return -ENOKEY;
if (type != &key_type_asymmetric)
return -EOPNOTSUPP;
sig = payload->data[asym_auth];
if (!sig)
return -ENOPKG;
if (!sig->auth_ids[0] && !sig->auth_ids[1] && !sig->auth_ids[2])
return -ENOKEY;
if (trusted) {
if (trusted->type == &key_type_keyring) {
/* See if we have a key that signed this one. */
key = find_asymmetric_key(trusted, sig->auth_ids[0],
sig->auth_ids[1],
sig->auth_ids[2], false);
if (IS_ERR(key))
key = NULL;
} else if (trusted->type == &key_type_asymmetric) {
const struct asymmetric_key_id **signer_ids;
signer_ids = (const struct asymmetric_key_id **)
asymmetric_key_ids(trusted)->id;
/*
* The auth_ids come from the candidate key (the
* one that is being considered for addition to
* dest_keyring) and identify the key that was
* used to sign.
*
* The signer_ids are identifiers for the
* signing key specified for dest_keyring.
*
* The first auth_id is the preferred id, 2nd and
* 3rd are the fallbacks. If exactly one of
* auth_ids[0] and auth_ids[1] is present, it may
* match either signer_ids[0] or signed_ids[1].
* If both are present the first one may match
* either signed_id but the second one must match
* the second signer_id. If neither of them is
* available, auth_ids[2] is matched against
* signer_ids[2] as a fallback.
*/
if (!sig->auth_ids[0] && !sig->auth_ids[1]) {
if (asymmetric_key_id_same(signer_ids[2],
sig->auth_ids[2]))
key = __key_get(trusted);
} else if (!sig->auth_ids[0] || !sig->auth_ids[1]) {
const struct asymmetric_key_id *auth_id;
auth_id = sig->auth_ids[0] ?: sig->auth_ids[1];
if (match_either_id(signer_ids, auth_id))
key = __key_get(trusted);
} else if (asymmetric_key_id_same(signer_ids[1],
sig->auth_ids[1]) &&
match_either_id(signer_ids,
sig->auth_ids[0])) {
key = __key_get(trusted);
}
} else {
return -EOPNOTSUPP;
}
}
if (check_dest && !key) {
/* See if the destination has a key that signed this one. */
key = find_asymmetric_key(dest_keyring, sig->auth_ids[0],
sig->auth_ids[1], sig->auth_ids[2],
false);
if (IS_ERR(key))
key = NULL;
}
if (!key)
return -ENOKEY;
ret = key_validate(key);
if (ret == 0)
ret = verify_signature(key, sig);
key_put(key);
return ret;
}
/**
* restrict_link_by_key_or_keyring - Restrict additions to a ring of public
* keys using the restrict_key information stored in the ring.
* @dest_keyring: Keyring being linked to.
* @type: The type of key being added.
* @payload: The payload of the new key.
* @trusted: A key or ring of keys that can be used to vouch for the new cert.
*
* Check the new certificate only against the key or keys passed in the data
* parameter. If one of those is the signing key and validates the new
* certificate, then mark the new certificate as being ok to link.
*
* Returns 0 if the new certificate was accepted, -ENOKEY if we
* couldn't find a matching parent certificate in the trusted list,
* -EKEYREJECTED if the signature check fails, -ENOPKG if the signature uses
* unsupported crypto, or some other error if there is a matching certificate
* but the signature check cannot be performed.
*/
int restrict_link_by_key_or_keyring(struct key *dest_keyring,
const struct key_type *type,
const union key_payload *payload,
struct key *trusted)
{
return key_or_keyring_common(dest_keyring, type, payload, trusted,
false);
}
/**
* restrict_link_by_key_or_keyring_chain - Restrict additions to a ring of
* public keys using the restrict_key information stored in the ring.
* @dest_keyring: Keyring being linked to.
* @type: The type of key being added.
* @payload: The payload of the new key.
* @trusted: A key or ring of keys that can be used to vouch for the new cert.
*
* Check the new certificate against the key or keys passed in the data
* parameter and against the keys already linked to the destination keyring. If
* one of those is the signing key and validates the new certificate, then mark
* the new certificate as being ok to link.
*
* Returns 0 if the new certificate was accepted, -ENOKEY if we
* couldn't find a matching parent certificate in the trusted list,
* -EKEYREJECTED if the signature check fails, -ENOPKG if the signature uses
* unsupported crypto, or some other error if there is a matching certificate
* but the signature check cannot be performed.
*/
int restrict_link_by_key_or_keyring_chain(struct key *dest_keyring,
const struct key_type *type,
const union key_payload *payload,
struct key *trusted)
{
return key_or_keyring_common(dest_keyring, type, payload, trusted,
true);
}