Re: [PATCH v8 5/5] certs: Allow root user to append signed hashes to the blacklist keyring

Mickaël Salaün
2022-03-08T16:02:37Z

On 08/03/2022 14:19, Jarkko Sakkinen wrote:
> On Tue, Mar 08, 2022 at 01:18:28PM +0100, Mickaël Salaün wrote:
>>
>> On 08/03/2022 12:53, Jarkko Sakkinen wrote:
>>> On Mon, Jul 12, 2021 at 07:03:13PM +0200, Mickaël Salaün wrote:
>>>> From: Mickaël Salaün <mic@linux.microsoft.com>
>>>>
>>>> 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@redhat.com>
>>>> Cc: David Woodhouse <dwmw2@infradead.org>
>>>> Cc: Eric Snowberg <eric.snowberg@oracle.com>
>>>> Cc: Jarkko Sakkinen <jarkko@kernel.org>
>>>> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
>>>> Link: https://lore.kernel.org/r/20210712170313.884724-6-mic@digikod.net
>>>> ---
>>>>
>>>> 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");
>>>
>>>
>>> This prevents me from taking this to my pull request. In moderns standards,
>>> no new BUG_ON(), panic() etc. should never added to the kernel.
>>>
>>> I missed this in my review.
>>>
>>> This should rather be e.g.
>>>
>>> restriction = kzalloc(sizeof(*restriction), GFP_KERNEL);
>>> if (!restriction) {
>>> pr_err("Can't allocate blacklist keyring restriction\n");
>>> return 0;
>>> }
>>>
>>> Unfortunately I need to drop this patch set, because adding new panic()
>>> is simply a no-go.
>>
>> I agree that panic() is not great in general, but I followed the other part
>> of the code (just above) that do the same. This part of the kernel should
>> failed if critical memory allocation failed at boot time (only). It doesn't
>> impact the kernel once it is running. I don't think that just ignoring this
>> error with return 0 is fine, after all it's a critical error right?
>
> It's not good reason enough to crash the whole kernel, even if it is a
> critical error (e.g. run-time foresincs). Even WARN() is not recommended
> these days [*].

I think that what Greg said in this email is that WARN*() should only be
used for cases that should never happen, it is definitely not
deprecated, but WARN_ON_ONCE() may be a better idea though. WARN*()
helps detect such thought-to-be-impossible cases, that can happen e.g.
with code refactoring.

A lot of initialization/boot code (e.g. without user space nor external
interactions, mostly __init functions) do panic if there is unexpected
and unrecoverable errors like failed memory allocations. I think
handling such errors otherwise would be more complex for no benefit.
Moreover, delegating such error handling to user space could create new
(silent) issues.

>
> For the existing panic()-statements: I'm happy to review patches that
> render them out. >
> Not sure tho, if this fails should it be then "everything blacklisted".
> Just one thing to consider.

Well, if it fail it will be "nothing will work afterwards". Do you have
a working and useful scenario for this kind of error?

>
>> Calling panic() seems OK here. Is there a better way to stop the kernel for
>> such critical error? If the kernel cannot allocate memory at this time, it
>> would be useless to try continuing booting.
>
> [*] https://lore.kernel.org/linux-sgx/YA0tvOGp%2FshchVhu@kroah.com/