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Date:   Tue, 12 Jan 2021 12:13:05 -0700
From:   Eric Snowberg <eric.snowberg@...cle.com>
To:     David Howells <dhowells@...hat.com>
Cc:     dwmw2@...radead.org,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        herbert@...dor.apana.org.au, davem@...emloft.net,
        jmorris@...ei.org, serge@...lyn.com, nayna@...ux.ibm.com,
        Mimi Zohar <zohar@...ux.ibm.com>, erichte@...ux.ibm.com,
        mpe@...erman.id.au, keyrings@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v4] certs: Add EFI_CERT_X509_GUID support for dbx entries


> On Jan 12, 2021, at 10:10 AM, David Howells <dhowells@...hat.com> wrote:
> 
> How about the attached?

This looks good to me.

> I've changed the function names to something that I
> think reads better, but otherwise it's the same.

I agree, the function name changes you made sound better.

We are starting to see platforms with KEK signed DBX updates containing
certs like this following boothole, so it would be great if we could get
something like this in.  I also had a follow on series that allowed these
certs to be compiled into the kernel.

https://lkml.org/lkml/2020/9/30/1301

I’d appreciate any feedback on that series as well.

Thanks

> David
> ---
> commit 8913866babb96fcfe452aac6042ca8862d4c0b53
> Author: Eric Snowberg <eric.snowberg@...cle.com>
> Date:   Tue Sep 15 20:49:27 2020 -0400
> 
>    certs: Add EFI_CERT_X509_GUID support for dbx entries
> 
>    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@...cle.com>
>    Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
>    Signed-off-by: David Howells <dhowells@...hat.com>
> 
> 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);
> +}
> +
> /*
>  * Return the appropriate handler for particular signature list types found in
>  * the UEFI db and MokListRT tables.
> @@ -76,5 +85,7 @@ __init efi_element_handler_t get_handler_for_dbx(const efi_guid_t *sig_type)
> 		return uefi_blacklist_x509_tbs;
> 	if (efi_guidcmp(*sig_type, efi_cert_sha256_guid) == 0)
> 		return uefi_blacklist_binary;
> +	if (efi_guidcmp(*sig_type, efi_cert_x509_guid) == 0)
> +		return uefi_revocation_list_x509;
> 	return 0;
> }
> 

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