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

Mickaël Salaün
2022-03-11T16:37:15Z

On 10/03/2022 00:11, Jarkko Sakkinen wrote:
> On Wed, Mar 09, 2022 at 07:36:50PM +0100, Mickaël Salaün wrote:
>>
>> On 09/03/2022 17:01, Jarkko Sakkinen wrote:
>>> On Tue, Mar 08, 2022 at 05:02:23PM +0100, Mickaël Salaün wrote:
>>>>
>>>> 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.
>>>
>>> To crash the whole kernel, you should be able to clearly explain why it
>>> makes sense in the situation.
>>
>> If there is no enough memory to allocate 24 bytes (struct key_restriction)
>> during early boot, I really doubt the kernel can do much.
>>
>>>
>>>>>
>>>>> 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?
>>>
>>> So you have zero chances to get a shell without blacklist just to do
>>> kernel forensics?
>>
>> Right, I don't think the kernel can launch any process (nor continue early
>> boot) if it cannot allocate 24 bytes.
>
> initcall is just wrong layer to choose to crash the kernel. It should be
> either do_initcall_level() or do_one_initcall() that should care about
> this (or not care). You can print error message and return -ENODEV;

Ok, I'll do that. Is it OK if I send you a patch fixing all panic calls
from blacklist_init() and system_trusted_keyring_init() to apply after
this series (with the panic call)?