Re: [PATCH v3 04/10] certs: Fix blacklist flag type confusion

Mickaël Salaün
2021-01-20T12:00:12Z

On 20/01/2021 04:55, Jarkko Sakkinen wrote:
> On Thu, Jan 14, 2021 at 04:19:03PM +0100, Mickaël Salaün wrote:
>> From: David Howells <dhowells@redhat.com>
>>
>> KEY_FLAG_KEEP is not meant to be passed to keyring_alloc() or key_alloc(),
>> as these only take KEY_ALLOC_* flags. KEY_FLAG_KEEP has the same value as
>> KEY_ALLOC_BYPASS_RESTRICTION, but fortunately only key_create_or_update()
>> uses it. LSMs using the key_alloc hook don't check that flag.
>>
>> KEY_FLAG_KEEP is then ignored but fortunately (again) the root user cannot
>> write to the blacklist keyring, so it is not possible to remove a key/hash
>> from it.
>>
>> Fix this by adding a KEY_ALLOC_SET_KEEP flag that tells key_alloc() to set
>> KEY_FLAG_KEEP on the new key. blacklist_init() can then, correctly, pass
>> this to keyring_alloc().
>
> OK, so thing work by luck now, but given the new patches which allow
> to append new keys they would break, right?

Without this fix, patch 9/10 would allow to remove and modify keys from
the blacklist keyring.

>
>> We can also use this in ima_mok_init() rather than setting the flag
>> manually.
>
> What does ima_mok_init() do?

This was initially an addition from David Howells, I only fixed the
argument bit-ORing. ima_mok_init() allocates a blacklist keyring (with
different properties) dedicated to IMA.

>> Note that this doesn't fix an observable bug with the current
>> implementation but it is required to allow addition of new hashes to the
>> blacklist in the future without making it possible for them to be removed.
>>
>> Fixes: 734114f8782f ("KEYS: Add a system blacklist keyring")
>> cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
>
> Nit: Cc

OK

>
>> Cc: David Woodhouse <dwmw2@infradead.org>
>> Reported-by: Mickaël Salaün <mic@linux.microsoft.com>
>> Signed-off-by: David Howells <dhowells@redhat.com>
>> [mic@linux.microsoft.com: fix ima_mok_init()]
>> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
>> ---
>>
>> Changes since v2:
>> * Cherry-pick rewritten v1 patch from
>> https://lore.kernel.org/lkml/2659836.1607940186@warthog.procyon.org.uk/
>> to rebase on v5.11-rc3 and fix ima_mok_init().
>> ---
>> certs/blacklist.c | 2 +-
>> include/linux/key.h | 1 +
>> security/integrity/ima/ima_mok.c | 4 +---
>> security/keys/key.c | 2 ++
>> 4 files changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/certs/blacklist.c b/certs/blacklist.c
>> index a888b934a1cd..029471947838 100644
>> --- a/certs/blacklist.c
>> +++ b/certs/blacklist.c
>> @@ -162,7 +162,7 @@ static int __init blacklist_init(void)
>> KEY_USR_VIEW | KEY_USR_READ |
>> KEY_USR_SEARCH,
>> KEY_ALLOC_NOT_IN_QUOTA |
>> - KEY_FLAG_KEEP,
>> + KEY_ALLOC_SET_KEEP,
>> NULL, NULL);
>> if (IS_ERR(blacklist_keyring))
>> panic("Can't allocate system blacklist keyring\n");
>> diff --git a/include/linux/key.h b/include/linux/key.h
>> index 0f2e24f13c2b..eed3ce139a32 100644
>> --- a/include/linux/key.h
>> +++ b/include/linux/key.h
>> @@ -289,6 +289,7 @@ extern struct key *key_alloc(struct key_type *type,
>> #define KEY_ALLOC_BUILT_IN 0x0004 /* Key is built into kernel */
>> #define KEY_ALLOC_BYPASS_RESTRICTION 0x0008 /* Override the check on restricted keyrings */
>> #define KEY_ALLOC_UID_KEYRING 0x0010 /* allocating a user or user session keyring */
>> +#define KEY_ALLOC_SET_KEEP 0x0020 /* Set the KEEP flag on the key/keyring */
>>
>> extern void key_revoke(struct key *key);
>> extern void key_invalidate(struct key *key);
>> diff --git a/security/integrity/ima/ima_mok.c b/security/integrity/ima/ima_mok.c
>> index 36cadadbfba4..5594dd38ab04 100644
>> --- a/security/integrity/ima/ima_mok.c
>> +++ b/security/integrity/ima/ima_mok.c
>> @@ -38,13 +38,11 @@ __init int ima_mok_init(void)
>> (KEY_POS_ALL & ~KEY_POS_SETATTR) |
>> KEY_USR_VIEW | KEY_USR_READ |
>> KEY_USR_WRITE | KEY_USR_SEARCH,
>> - KEY_ALLOC_NOT_IN_QUOTA,
>> + KEY_ALLOC_NOT_IN_QUOTA | KEY_ALLOC_SET_KEEP,
>> restriction, NULL);
>>
>> if (IS_ERR(ima_blacklist_keyring))
>> panic("Can't allocate IMA blacklist keyring.");
>> -
>> - set_bit(KEY_FLAG_KEEP, &ima_blacklist_keyring->flags);
>> return 0;
>> }
>> device_initcall(ima_mok_init);
>> diff --git a/security/keys/key.c b/security/keys/key.c
>> index ebe752b137aa..c45afdd1dfbb 100644
>> --- a/security/keys/key.c
>> +++ b/security/keys/key.c
>> @@ -303,6 +303,8 @@ struct key *key_alloc(struct key_type *type, const char *desc,
>> key->flags |= 1 << KEY_FLAG_BUILTIN;
>> if (flags & KEY_ALLOC_UID_KEYRING)
>> key->flags |= 1 << KEY_FLAG_UID_KEYRING;
>> + if (flags & KEY_ALLOC_SET_KEEP)
>> + key->flags |= 1 << KEY_FLAG_KEEP;
>>
>> #ifdef KEY_DEBUGGING
>> key->magic = KEY_DEBUG_MAGIC;
>> --
>> 2.30.0
>>
>
> Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
>
> /Jarkko
>