Re: [PATCH v5 1/4] certs: Add EFI_CERT_X509_GUID support for dbx entries

Nayna
2021-01-28T04:50:34Z

On 1/22/21 1:10 PM, Eric Snowberg wrote:
> This fixes CVE-2020-26541.
>
> The Secure Boot Forbidden Signature Database, dbx, contains a list of now
> revoked signatures and keys previously approved to boot with UEFI Secure
> Boot enabled. The dbx is capable of containing any number of
> EFI_CERT_X509_SHA256_GUID, EFI_CERT_SHA256_GUID, and EFI_CERT_X509_GUID
> entries.
>
> Currently when EFI_CERT_X509_GUID are contained in the dbx, the entries are
> skipped.
>
> Add support for EFI_CERT_X509_GUID dbx entries. When a EFI_CERT_X509_GUID
> is found, it is added as an asymmetrical key to the .blacklist keyring.
> Anytime the .platform keyring is used, the keys in the .blacklist keyring
> are referenced, if a matching key is found, the key will be rejected.
>
> Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
> Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
> v5: Function name changes done by David Howells
> ---
> certs/blacklist.c | 32 +++++++++++++++++++
> certs/blacklist.h | 12 +++++++
> certs/system_keyring.c | 6 ++++
> include/keys/system_keyring.h | 11 +++++++
> .../platform_certs/keyring_handler.c | 11 +++++++
> 5 files changed, 72 insertions(+)
>
> diff --git a/certs/blacklist.c b/certs/blacklist.c
> index 6514f9ebc943..a7f021878a4b 100644
> --- a/certs/blacklist.c
> +++ b/certs/blacklist.c
> @@ -100,6 +100,38 @@ int mark_hash_blacklisted(const char *hash)
> return 0;
> }
>
> +int add_key_to_revocation_list(const char *data, size_t size)
> +{
> + key_ref_t key;
> +
> + key = key_create_or_update(make_key_ref(blacklist_keyring, true),
> + "asymmetric",
> + NULL,
> + data,
> + size,
> + ((KEY_POS_ALL & ~KEY_POS_SETATTR) | KEY_USR_VIEW),
> + KEY_ALLOC_NOT_IN_QUOTA | KEY_ALLOC_BUILT_IN);
> +
> + if (IS_ERR(key)) {
> + pr_err("Problem with revocation key (%ld)\n", PTR_ERR(key));
> + return PTR_ERR(key);
> + }
> +
> + return 0;
> +}
> +
> +int is_key_on_revocation_list(struct pkcs7_message *pkcs7)
> +{
> + int ret;
> +
> + ret = validate_trust(pkcs7, blacklist_keyring);
> +
> + if (ret == 0)
> + return -EKEYREJECTED;
> +
> + return -ENOKEY;
> +}
> +
> /**
> * is_hash_blacklisted - Determine if a hash is blacklisted
> * @hash: The hash to be checked as a binary blob
> diff --git a/certs/blacklist.h b/certs/blacklist.h
> index 1efd6fa0dc60..420bb7c86e07 100644
> --- a/certs/blacklist.h
> +++ b/certs/blacklist.h
> @@ -1,3 +1,15 @@
> #include <linux/kernel.h>
> +#include <linux/errno.h>
> +#include <crypto/pkcs7.h>
>
> extern const char __initconst *const blacklist_hashes[];
> +
> +#ifdef CONFIG_INTEGRITY_PLATFORM_KEYRING
> +#define validate_trust pkcs7_validate_trust
> +#else
> +static inline int validate_trust(struct pkcs7_message *pkcs7,
> + struct key *trust_keyring)
> +{
> + return -ENOKEY;
> +}
> +#endif
> diff --git a/certs/system_keyring.c b/certs/system_keyring.c
> index 798291177186..cc165b359ea3 100644
> --- a/certs/system_keyring.c
> +++ b/certs/system_keyring.c
> @@ -241,6 +241,12 @@ int verify_pkcs7_message_sig(const void *data, size_t len,
> pr_devel("PKCS#7 platform keyring is not available\n");
> goto error;
> }
> +
> + ret = is_key_on_revocation_list(pkcs7);
> + if (ret != -ENOKEY) {
> + pr_devel("PKCS#7 platform key is on revocation list\n");
> + goto error;
> + }
> }
> ret = pkcs7_validate_trust(pkcs7, trusted_keys);
> if (ret < 0) {
> diff --git a/include/keys/system_keyring.h b/include/keys/system_keyring.h
> index fb8b07daa9d1..61f98739e8b1 100644
> --- a/include/keys/system_keyring.h
> +++ b/include/keys/system_keyring.h
> @@ -31,11 +31,14 @@ extern int restrict_link_by_builtin_and_secondary_trusted(
> #define restrict_link_by_builtin_and_secondary_trusted restrict_link_by_builtin_trusted
> #endif
>
> +extern struct pkcs7_message *pkcs7;
> #ifdef CONFIG_SYSTEM_BLACKLIST_KEYRING
> extern int mark_hash_blacklisted(const char *hash);
> +extern int add_key_to_revocation_list(const char *data, size_t size);
> extern int is_hash_blacklisted(const u8 *hash, size_t hash_len,
> const char *type);
> extern int is_binary_blacklisted(const u8 *hash, size_t hash_len);
> +extern int is_key_on_revocation_list(struct pkcs7_message *pkcs7);
> #else
> static inline int is_hash_blacklisted(const u8 *hash, size_t hash_len,
> const char *type)
> @@ -47,6 +50,14 @@ static inline int is_binary_blacklisted(const u8 *hash, size_t hash_len)
> {
> return 0;
> }
> +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)
> +{
> + return -ENOKEY;
> +}
> #endif
>
> #ifdef CONFIG_IMA_BLACKLIST_KEYRING
> diff --git a/security/integrity/platform_certs/keyring_handler.c b/security/integrity/platform_certs/keyring_handler.c
> index c5ba695c10e3..5604bd57c990 100644
> --- a/security/integrity/platform_certs/keyring_handler.c
> +++ b/security/integrity/platform_certs/keyring_handler.c
> @@ -55,6 +55,15 @@ static __init void uefi_blacklist_binary(const char *source,
> uefi_blacklist_hash(source, data, len, "bin:", 4);
> }
>
> +/*
> + * Add an X509 cert to the revocation list.
> + */
> +static __init void uefi_revocation_list_x509(const char *source,
> + const void *data, size_t len)
> +{
> + add_key_to_revocation_list(data, len);
> +}

In keeping the naming convention with other functions that blacklist
hashes, why can't we call these functions:

* uefi_revocation_list_x509() -> uefi_blacklist_x509_cert()
* add_key_to_revocation_list() -> uefi_blacklist_cert()
* is_key_on_revocation_list() -> is_cert_blacklisted()

Thanks & Regards,

- Nayna