From: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx> Add a kernel option SYSTEM_BLACKLIST_AUTH_UPDATE to enable the root user to dynamically add new keys to the blacklist keyring. This enables to invalidate new certificates, either from being loaded in a keyring, or from being trusted in a PKCS#7 certificate chain. This also enables to add new file hashes to be denied by the integrity infrastructure. Being able to untrust a certificate which could have normaly been trusted is a sensitive operation. This is why adding new hashes to the blacklist keyring is only allowed when these hashes are signed and vouched by the builtin trusted keyring. A blacklist hash is stored as a key description. The PKCS#7 signature of this description must be provided as the key payload. Marking a certificate as untrusted should be enforced while the system is running. It is then forbiden to remove such blacklist keys. Update blacklist keyring, blacklist key and revoked certificate access rights: * allows the root user to search for a specific blacklisted hash, which make sense because the descriptions are already viewable; * forbids key update (blacklist and asymmetric ones); * restricts kernel rights on the blacklist keyring to align with the root user rights. See help in tools/certs/print-cert-tbs-hash.sh . Cc: David Howells <dhowells@xxxxxxxxxx> Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx> Cc: Eric Snowberg <eric.snowberg@xxxxxxxxxx> Cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx> Link: https://lore.kernel.org/r/20210712170313.884724-6-mic@xxxxxxxxxxx --- Changes since v6: * Rebase on keys-cve-2020-26541-v3: commit ebd9c2ae369a ("integrity: Load mokx variables into the blacklist keyring"). Changes since v5: * Rebase on keys-next, fix Kconfig conflict, and update the asymmetric key rights added to the blacklist keyring by the new add_key_to_revocation_list(): align with blacklist key rights by removing KEY_POS_WRITE as a safeguard, and add KEY_ALLOC_BYPASS_RESTRICTION to not be subject to restrict_link_for_blacklist() that only allows blacklist key types to be added to the keyring. * Change the return code for restrict_link_for_blacklist() from -EPERM to -EOPNOTSUPP to align with asymmetric key keyrings. Changes since v3: * Update commit message for print-cert-tbs-hash.sh . Changes since v2: * Add comment for blacklist_key_instantiate(). --- certs/Kconfig | 10 +++++ certs/blacklist.c | 96 ++++++++++++++++++++++++++++++++++++----------- 2 files changed, 85 insertions(+), 21 deletions(-) diff --git a/certs/Kconfig b/certs/Kconfig index 0fbe184ceca5..e0e524b7eff9 100644 --- a/certs/Kconfig +++ b/certs/Kconfig @@ -103,4 +103,14 @@ config SYSTEM_REVOCATION_KEYS containing X.509 certificates to be included in the default blacklist keyring. +config SYSTEM_BLACKLIST_AUTH_UPDATE + bool "Allow root to add signed blacklist keys" + depends on SYSTEM_BLACKLIST_KEYRING + depends on SYSTEM_DATA_VERIFICATION + help + If set, provide the ability to load new blacklist keys at run time if + they are signed and vouched by a certificate from the builtin trusted + keyring. The PKCS#7 signature of the description is set in the key + payload. Blacklist keys cannot be removed. + endmenu diff --git a/certs/blacklist.c b/certs/blacklist.c index b254c87ceb3a..486ce0dd8e9c 100644 --- a/certs/blacklist.c +++ b/certs/blacklist.c @@ -15,6 +15,7 @@ #include <linux/err.h> #include <linux/seq_file.h> #include <linux/uidgid.h> +#include <linux/verification.h> #include <keys/system_keyring.h> #include "blacklist.h" #include "common.h" @@ -26,6 +27,9 @@ */ #define MAX_HASH_LEN 128 +#define BLACKLIST_KEY_PERM (KEY_POS_SEARCH | KEY_POS_VIEW | \ + KEY_USR_SEARCH | KEY_USR_VIEW) + static const char tbs_prefix[] = "tbs"; static const char bin_prefix[] = "bin"; @@ -80,19 +84,51 @@ static int blacklist_vet_description(const char *desc) return 0; } -/* - * The hash to be blacklisted is expected to be in the description. There will - * be no payload. - */ -static int blacklist_preparse(struct key_preparsed_payload *prep) +static int blacklist_key_instantiate(struct key *key, + struct key_preparsed_payload *prep) { - if (prep->datalen > 0) - return -EINVAL; - return 0; +#ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE + int err; +#endif + + /* Sets safe default permissions for keys loaded by user space. */ + key->perm = BLACKLIST_KEY_PERM; + + /* + * Skips the authentication step for builtin hashes, they are not + * signed but still trusted. + */ + if (key->flags & (1 << KEY_FLAG_BUILTIN)) + goto out; + +#ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE + /* + * Verifies the description's PKCS#7 signature against the builtin + * trusted keyring. + */ + err = verify_pkcs7_signature(key->description, + strlen(key->description), prep->data, prep->datalen, + NULL, VERIFYING_UNSPECIFIED_SIGNATURE, NULL, NULL); + if (err) + return err; +#else + /* + * It should not be possible to come here because the keyring doesn't + * have KEY_USR_WRITE and the only other way to call this function is + * for builtin hashes. + */ + WARN_ON_ONCE(1); + return -EPERM; +#endif + +out: + return generic_key_instantiate(key, prep); } -static void blacklist_free_preparse(struct key_preparsed_payload *prep) +static int blacklist_key_update(struct key *key, + struct key_preparsed_payload *prep) { + return -EPERM; } static void blacklist_describe(const struct key *key, struct seq_file *m) @@ -103,9 +139,8 @@ static void blacklist_describe(const struct key *key, struct seq_file *m) static struct key_type key_type_blacklist = { .name = "blacklist", .vet_description = blacklist_vet_description, - .preparse = blacklist_preparse, - .free_preparse = blacklist_free_preparse, - .instantiate = generic_key_instantiate, + .instantiate = blacklist_key_instantiate, + .update = blacklist_key_update, .describe = blacklist_describe, }; @@ -154,8 +189,7 @@ static int mark_raw_hash_blacklisted(const char *hash) hash, NULL, 0, - ((KEY_POS_ALL & ~KEY_POS_SETATTR) | - KEY_USR_VIEW), + BLACKLIST_KEY_PERM, KEY_ALLOC_NOT_IN_QUOTA | KEY_ALLOC_BUILT_IN); if (IS_ERR(key)) { @@ -232,8 +266,10 @@ int add_key_to_revocation_list(const char *data, size_t size) NULL, data, size, - ((KEY_POS_ALL & ~KEY_POS_SETATTR) | KEY_USR_VIEW), - KEY_ALLOC_NOT_IN_QUOTA | KEY_ALLOC_BUILT_IN); + KEY_POS_VIEW | KEY_POS_READ | KEY_POS_SEARCH + | KEY_USR_VIEW, + KEY_ALLOC_NOT_IN_QUOTA | KEY_ALLOC_BUILT_IN + | KEY_ALLOC_BYPASS_RESTRICTION); if (IS_ERR(key)) { pr_err("Problem with revocation key (%ld)\n", PTR_ERR(key)); @@ -260,25 +296,43 @@ int is_key_on_revocation_list(struct pkcs7_message *pkcs7) } #endif +static int restrict_link_for_blacklist(struct key *dest_keyring, + const struct key_type *type, const union key_payload *payload, + struct key *restrict_key) +{ + if (type == &key_type_blacklist) + return 0; + return -EOPNOTSUPP; +} + /* * Initialise the blacklist */ static int __init blacklist_init(void) { const char *const *bl; + struct key_restriction *restriction; if (register_key_type(&key_type_blacklist) < 0) panic("Can't allocate system blacklist key type\n"); + restriction = kzalloc(sizeof(*restriction), GFP_KERNEL); + if (!restriction) + panic("Can't allocate blacklist keyring restriction\n"); + restriction->check = restrict_link_for_blacklist; + blacklist_keyring = keyring_alloc(".blacklist", GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, current_cred(), - (KEY_POS_ALL & ~KEY_POS_SETATTR) | - KEY_USR_VIEW | KEY_USR_READ | - KEY_USR_SEARCH, - KEY_ALLOC_NOT_IN_QUOTA | + KEY_POS_VIEW | KEY_POS_READ | KEY_POS_SEARCH | + KEY_POS_WRITE | + KEY_USR_VIEW | KEY_USR_READ | KEY_USR_SEARCH +#ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE + | KEY_USR_WRITE +#endif + , KEY_ALLOC_NOT_IN_QUOTA | KEY_ALLOC_SET_KEEP, - NULL, NULL); + restriction, NULL); if (IS_ERR(blacklist_keyring)) panic("Can't allocate system blacklist keyring\n"); -- 2.32.0
