Re: [PATCH v9,2/4] PKCS#7: Check codeSigning EKU for kernel module and kexec pe verification

From: Jarkko Sakkinen
Date: Sat Aug 27 2022 - 23:34:44 EST

On Thu, Aug 25, 2022 at 10:23:12PM +0800, Lee, Chun-Yi wrote:
> This patch adds the logic for checking the CodeSigning extended
> key usage when verifying signature of kernel module or
> kexec PE binary in PKCS#7.

Pretty much the same feedback as for 1/4.

>
> Signed-off-by: "Lee, Chun-Yi" <jlee@xxxxxxxx>
> ---
> certs/blacklist.c | 5 ++--
> certs/system_keyring.c | 4 +--
> crypto/asymmetric_keys/Kconfig | 9 ++++++
> crypto/asymmetric_keys/pkcs7_trust.c | 43 ++++++++++++++++++++++++++--
> crypto/asymmetric_keys/selftest.c | 2 +-
> include/crypto/pkcs7.h | 4 ++-
> include/keys/system_keyring.h | 7 +++--
> 7 files changed, 63 insertions(+), 11 deletions(-)
>
> diff --git a/certs/blacklist.c b/certs/blacklist.c
> index 41f10601cc72..fa41454055be 100644
> --- a/certs/blacklist.c
> +++ b/certs/blacklist.c
> @@ -282,11 +282,12 @@ int add_key_to_revocation_list(const char *data, size_t size)
> * is_key_on_revocation_list - Determine if the key for a PKCS#7 message is revoked
> * @pkcs7: The PKCS#7 message to check
> */
> -int is_key_on_revocation_list(struct pkcs7_message *pkcs7)
> +int is_key_on_revocation_list(struct pkcs7_message *pkcs7,
> + enum key_being_used_for usage)
> {
> int ret;
>
> - ret = pkcs7_validate_trust(pkcs7, blacklist_keyring);
> + ret = pkcs7_validate_trust(pkcs7, blacklist_keyring, usage, false);
>
> if (ret == 0)
> return -EKEYREJECTED;
> diff --git a/certs/system_keyring.c b/certs/system_keyring.c
> index 5042cc54fa5e..66737bfb26de 100644
> --- a/certs/system_keyring.c
> +++ b/certs/system_keyring.c
> @@ -263,13 +263,13 @@ int verify_pkcs7_message_sig(const void *data, size_t len,
> goto error;
> }
>
> - ret = is_key_on_revocation_list(pkcs7);
> + ret = is_key_on_revocation_list(pkcs7, usage);
> if (ret != -ENOKEY) {
> pr_devel("PKCS#7 platform key is on revocation list\n");
> goto error;
> }
> }
> - ret = pkcs7_validate_trust(pkcs7, trusted_keys);
> + ret = pkcs7_validate_trust(pkcs7, trusted_keys, usage, true);
> if (ret < 0) {
> if (ret == -ENOKEY)
> pr_devel("PKCS#7 signature not signed with a trusted key\n");
> diff --git a/crypto/asymmetric_keys/Kconfig b/crypto/asymmetric_keys/Kconfig
> index 3df3fe4ed95f..189536bd0f9a 100644
> --- a/crypto/asymmetric_keys/Kconfig
> +++ b/crypto/asymmetric_keys/Kconfig
> @@ -85,4 +85,13 @@ config FIPS_SIGNATURE_SELFTEST
> depends on ASYMMETRIC_KEY_TYPE
> depends on PKCS7_MESSAGE_PARSER
>
> +config CHECK_CODESIGN_EKU
> + bool "Check codeSigning extended key usage"
> + depends on PKCS7_MESSAGE_PARSER=y
> + depends on SYSTEM_DATA_VERIFICATION
> + help
> + This option provides support for checking the codeSigning extended
> + key usage when verifying the signature in PKCS#7. It affects kernel
> + module verification and kexec PE binary verification.
> +
> endif # ASYMMETRIC_KEY_TYPE
> diff --git a/crypto/asymmetric_keys/pkcs7_trust.c b/crypto/asymmetric_keys/pkcs7_trust.c
> index 9a87c34ed173..087d3761d9a8 100644
> --- a/crypto/asymmetric_keys/pkcs7_trust.c
> +++ b/crypto/asymmetric_keys/pkcs7_trust.c
> @@ -16,12 +16,40 @@
> #include <crypto/public_key.h>
> #include "pkcs7_parser.h"
>
> +#ifdef CONFIG_CHECK_CODESIGN_EKU
> +static bool check_eku_by_usage(struct key *key, enum key_being_used_for usage)
> +{
> + struct public_key *public_key = key->payload.data[asym_crypto];
> + bool ret = true;
> +
> + switch (usage) {
> + case VERIFYING_MODULE_SIGNATURE:
> + case VERIFYING_KEXEC_PE_SIGNATURE:
> + ret = !!(public_key->ext_key_usage & EKU_codeSigning);
> + if (!ret)
> + pr_warn("The signer '%s' key is not CodeSigning\n",
> + key->description);
> + break;
> + default:
> + break;
> + }
> + return ret;
> +}
> +#else
> +static bool check_eku_by_usage(struct key *key, enum key_being_used_for usage)
> +{
> + return true;
> +}
> +#endif
> +
> /*
> * Check the trust on one PKCS#7 SignedInfo block.
> */
> static int pkcs7_validate_trust_one(struct pkcs7_message *pkcs7,
> struct pkcs7_signed_info *sinfo,
> - struct key *trust_keyring)
> + struct key *trust_keyring,
> + enum key_being_used_for usage,
> + bool check_eku)
> {
> struct public_key_signature *sig = sinfo->sig;
> struct x509_certificate *x509, *last = NULL, *p;
> @@ -112,6 +140,10 @@ static int pkcs7_validate_trust_one(struct pkcs7_message *pkcs7,
> return -ENOKEY;
>
> matched:
> + if (check_eku && !check_eku_by_usage(key, usage)) {
> + key_put(key);
> + return -ENOKEY;
> + }

Why you don't just open code this to those rare call
sites where it is needed?

I counted that there is exactly one call site.

> ret = verify_signature(key, sig);
> key_put(key);
> if (ret < 0) {
> @@ -135,6 +167,8 @@ static int pkcs7_validate_trust_one(struct pkcs7_message *pkcs7,
> * pkcs7_validate_trust - Validate PKCS#7 trust chain
> * @pkcs7: The PKCS#7 certificate to validate
> * @trust_keyring: Signing certificates to use as starting points
> + * @usage: The use to which the key is being put.
> + * @check_eku: Check EKU (Extended Key Usage)
> *
> * Validate that the certificate chain inside the PKCS#7 message intersects
> * keys we already know and trust.
> @@ -156,7 +190,9 @@ static int pkcs7_validate_trust_one(struct pkcs7_message *pkcs7,
> * May also return -ENOMEM.
> */
> int pkcs7_validate_trust(struct pkcs7_message *pkcs7,
> - struct key *trust_keyring)
> + struct key *trust_keyring,
> + enum key_being_used_for usage,
> + bool check_eku)
> {
> struct pkcs7_signed_info *sinfo;
> struct x509_certificate *p;
> @@ -167,7 +203,8 @@ int pkcs7_validate_trust(struct pkcs7_message *pkcs7,
> p->seen = false;
>
> for (sinfo = pkcs7->signed_infos; sinfo; sinfo = sinfo->next) {
> - ret = pkcs7_validate_trust_one(pkcs7, sinfo, trust_keyring);
> + ret = pkcs7_validate_trust_one(pkcs7, sinfo, trust_keyring,
> + usage, check_eku);
> switch (ret) {
> case -ENOKEY:
> continue;
> diff --git a/crypto/asymmetric_keys/selftest.c b/crypto/asymmetric_keys/selftest.c
> index fa0bf7f24284..756e9f224d8a 100644
> --- a/crypto/asymmetric_keys/selftest.c
> +++ b/crypto/asymmetric_keys/selftest.c
> @@ -212,7 +212,7 @@ int __init fips_signature_selftest(void)
> if (ret < 0)
> panic("Certs selftest %d: pkcs7_verify() = %d\n", i, ret);
>
> - ret = pkcs7_validate_trust(pkcs7, keyring);
> + ret = pkcs7_validate_trust(pkcs7, keyring, VERIFYING_MODULE_SIGNATURE, false);
> if (ret < 0)
> panic("Certs selftest %d: pkcs7_validate_trust() = %d\n", i, ret);
>
> diff --git a/include/crypto/pkcs7.h b/include/crypto/pkcs7.h
> index 38ec7f5f9041..5d87b8a02f79 100644
> --- a/include/crypto/pkcs7.h
> +++ b/include/crypto/pkcs7.h
> @@ -30,7 +30,9 @@ extern int pkcs7_get_content_data(const struct pkcs7_message *pkcs7,
> * pkcs7_trust.c
> */
> extern int pkcs7_validate_trust(struct pkcs7_message *pkcs7,
> - struct key *trust_keyring);
> + struct key *trust_keyring,
> + enum key_being_used_for usage,
> + bool check_eku);
>
> /*
> * pkcs7_verify.c
> diff --git a/include/keys/system_keyring.h b/include/keys/system_keyring.h
> index 91e080efb918..bb33b527240e 100644
> --- a/include/keys/system_keyring.h
> +++ b/include/keys/system_keyring.h
> @@ -9,6 +9,7 @@
> #define _KEYS_SYSTEM_KEYRING_H
>
> #include <linux/key.h>
> +#include <keys/asymmetric-type.h>
>
> enum blacklist_hash_type {
> /* TBSCertificate hash */
> @@ -81,13 +82,15 @@ static inline int is_binary_blacklisted(const u8 *hash, size_t hash_len)
>
> #ifdef CONFIG_SYSTEM_REVOCATION_LIST
> extern int add_key_to_revocation_list(const char *data, size_t size);
> -extern int is_key_on_revocation_list(struct pkcs7_message *pkcs7);
> +extern int is_key_on_revocation_list(struct pkcs7_message *pkcs7,
> + enum key_being_used_for usage);
> #else
> static inline int add_key_to_revocation_list(const char *data, size_t size)
> {
> return 0;
> }
> -static inline int is_key_on_revocation_list(struct pkcs7_message *pkcs7)
> +static inline int is_key_on_revocation_list(struct pkcs7_message *pkcs7,
> + enum key_being_used_for usage)
> {
> return -ENOKEY;
> }
> --
> 2.26.2
>

BR, Jarkko