Re: [PATCH 8/8] ima: Differentiate auditing policy rules from "audit" actions

[Paul Moore]

On Fri, Jun 1, 2018 at 4:00 PM, Stefan Berger
<stefanb@xxxxxxxxxxxxxxxxxx> wrote:
> On 05/30/2018 07:34 PM, Richard Guy Briggs wrote:
>>
>> On 2018-05-30 17:38, Stefan Berger wrote:
>>>
>>> On 05/30/2018 05:22 PM, Paul Moore wrote:
>>>>
>>>> On Wed, May 30, 2018 at 9:08 AM, Stefan Berger
>>>> <stefanb@xxxxxxxxxxxxxxxxxx> wrote:
>>>>>
>>>>> On 05/30/2018 08:49 AM, Richard Guy Briggs wrote:
>>>>>>
>>>>>> On 2018-05-24 16:11, Stefan Berger wrote:
>>>>>>>
>>>>>>> The AUDIT_INTEGRITY_RULE is used for auditing IMA policy rules and
>>>>>>> the IMA "audit" policy action.  This patch defines
>>>>>>> AUDIT_INTEGRITY_POLICY_RULE to reflect the IMA policy rules.
>>>>>>>
>>>>>>> With this change we now call integrity_audit_msg_common() to get
>>>>>>> common integrity auditing fields. This now produces the following
>>>>>>> record when parsing an IMA policy rule:
>>>>>>>
>>>>>>> type=UNKNOWN[1806] msg=audit(1527004216.690:311): action=dont_measure
>>>>>>> \
>>>>>>>      fsmagic=0x9fa0 pid=1613 uid=0 auid=0 ses=2 \
>>>>>>>      subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 \
>>>>>>>      op=policy_update cause=parse_rule comm="echo"
>>>>>>> exe="/usr/bin/echo" \
>>>>>>>      tty=tty2 res=1
>>>>>>>
>>>>>>> Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxxxxxxx>
>>>>>>> ---
>>>>>>>     include/uapi/linux/audit.h          | 3 ++-
>>>>>>>     security/integrity/ima/ima_policy.c | 5 +++--
>>>>>>>     2 files changed, 5 insertions(+), 3 deletions(-)
>>>>>>>
>>>>>>> diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
>>>>>>> index 4e61a9e05132..776e0abd35cf 100644
>>>>>>> --- a/include/uapi/linux/audit.h
>>>>>>> +++ b/include/uapi/linux/audit.h
>>>>>>> @@ -146,7 +146,8 @@
>>>>>>>     #define AUDIT_INTEGRITY_STATUS            1802 /* Integrity
>>>>>>> enable
>>>>>>> status */
>>>>>>>     #define AUDIT_INTEGRITY_HASH      1803 /* Integrity HASH type */
>>>>>>>     #define AUDIT_INTEGRITY_PCR       1804 /* PCR invalidation msgs
>>>>>>> */
>>>>>>> -#define AUDIT_INTEGRITY_RULE       1805 /* policy rule */
>>>>>>> +#define AUDIT_INTEGRITY_RULE       1805 /* IMA "audit" action policy
>>>>>>> msgs  */
>>>>>>> +#define AUDIT_INTEGRITY_POLICY_RULE 1806 /* IMA policy rules */
>>>>>>>       #define AUDIT_KERNEL                2000    /* Asynchronous
>>>>>>> audit
>>>>>>> record. NOT A REQUEST. */
>>>>>>>     diff --git a/security/integrity/ima/ima_policy.c
>>>>>>> b/security/integrity/ima/ima_policy.c
>>>>>>> index 3aed25a7178a..a8ae47a386b4 100644
>>>>>>> --- a/security/integrity/ima/ima_policy.c
>>>>>>> +++ b/security/integrity/ima/ima_policy.c
>>>>>>> @@ -634,7 +634,7 @@ static int ima_parse_rule(char *rule, struct
>>>>>>> ima_rule_entry *entry)
>>>>>>>           int result = 0;
>>>>>>>           ab = integrity_audit_log_start(NULL, GFP_KERNEL,
>>>>>>> -                                      AUDIT_INTEGRITY_RULE);
>>>>>>> +                                      AUDIT_INTEGRITY_POLICY_RULE);
>>>>>>
>>>>>> Is it possible to connect this record to a syscall by replacing the
>>>>>> first parameter (NULL) by current->context?
>>>>
>>>> We're likely going to need to "associate" this record (audit speak for
>>>> making the first parameter non-NULL) with others for the audit
>>>> container ID work.  If you do it now, Richard's patches will likely
>>>> get a few lines smaller and that will surely make him a bit happier :)
>>>
>>> Richard is also introducing a local context that we can then create and
>>> use
>>> instead of the NULL. Can we not use that then?
>>
>> That is for records for which there is no syscall or user associated.
>>
>> In fact there is another recent change that would be better to use than
>> current->audit_context, which is the function audit_context().
>> See commit cdfb6b3 ("audit: use inline function to get audit context").
>>
>>> Steven seems to say: "We don't want to add syscall records to everything.
>>> That messes up schemas and existing code. The integrity events are 1
>>> record
>>> in size and should stay that way. This saves disk space and improves
>>> readability."
>>>
>>>>> We would have to fix current->context in this case since it is NULL. We
>>>>> get
>>>>> to this location by root cat'ing a policy or writing a policy filename
>>>>> into
>>>>> /sys/kernel/security/ima/policy.
>>>>
>>>> Perhaps I'm missing something, but current in this case should point
>>>> to the process which is writing to the policy file, yes?
>>>
>>> Yes, but current->context is NULL for some reason.
>>
>> Is it always this way?  If it isn't, which it should not be, we should
>> find out why.  Well, we should find out why this is NULL anyways, since
>> it shouldn't be.
>
>
> When someone writes a policy for IMA into securityfs, it's always NULL.
> There's another location where IMA uses the current->audit_context, and
> that's here:
>
> https://elixir.bootlin.com/linux/latest/source/security/integrity/ima/ima_api.c#L323
>
> At this location we sometimes see a (background) process with an
> audit_context but in the majority of cases it's current->audit_context is
> NULL. Starting a process as root or also non-root user, with the appropriate
> IMA audit policy rules set, we always see a NULL audit_context here.

What does your audit configuration look like?

Depending on your configuration a NULL audit_context can be expected,
see audit_dummy_context().  I believe the default Fedora audit config
will leave you with a NULL audit_context for all processes.  I also
believe that unless you explicitly set "audit=1" on the kernel command
line the init/systemd process will have a NULL audit_context (there
was actually a range of kernels where even setting "audit=1" wouldn't
be sufficient due to a bug we fixed a little while ago).

Look at the audit_alloc() function, it is called when a new process is
fork'd and is responsible for allocating a new audit_context.  If the
currently loaded audit config dictates that auditing is to be disabled
for this new process (state == AUDIT_DISABLED) then an audit_context
is not allocated and current->context remains NULL.

-- 
paul moore
www.paul-moore.com