Re: [PATCH v4 10/16] ima: Implement hierarchical processing of file accesses

From: Stefan Berger <stefanb@linux.ibm.com>
To: Mimi Zohar <zohar@linux.ibm.com>,
	Christian Brauner <christian.brauner@ubuntu.com>
Cc: linux-integrity@vger.kernel.org, serge@hallyn.com,
	containers@lists.linux.dev, dmitry.kasatkin@gmail.com,
	ebiederm@xmission.com, krzysztof.struczynski@huawei.com,
	roberto.sassu@huawei.com, mpeters@redhat.com, lhinds@redhat.com,
	lsturman@redhat.com, puiterwi@redhat.com, jejb@linux.ibm.com,
	jamjoom@us.ibm.com, linux-kernel@vger.kernel.org,
	paul@paul-moore.com, rgb@redhat.com,
	linux-security-module@vger.kernel.org, jmorris@namei.org
Subject: Re: [PATCH v4 10/16] ima: Implement hierarchical processing of file accesses
Date: Wed, 15 Dec 2021 21:55:35 -0500	[thread overview]
Message-ID: <cd14c566-afaf-8295-5445-420164903336@linux.ibm.com> (raw)
In-Reply-To: <2cd00e8d0fa819b507cabd34e8f1760d5ff783c6.camel@linux.ibm.com>

On 12/15/21 18:04, Mimi Zohar wrote:
> On Wed, 2021-12-08 at 13:22 -0500, Stefan Berger wrote:
>> On 12/8/21 11:50, Stefan Berger wrote:
>>> On 12/8/21 07:23, Christian Brauner wrote:
>>>> On Wed, Dec 08, 2021 at 01:09:54PM +0100, Christian Brauner wrote:
>>>>> On Tue, Dec 07, 2021 at 03:21:21PM -0500, Stefan Berger wrote:
>>>>>> Implement hierarchical processing of file accesses in IMA
>>>>>> namespaces by
>>>>>> walking the list of IMA namespaces towards the init_ima_ns. This way
>>>>>> file accesses can be audited in an IMA namespace and also be evaluated
>>>>>> against the IMA policies of parent IMA namespaces.
>>>>>>
>>>>>> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
>>>>>> ---
>>>>>>    security/integrity/ima/ima_main.c | 29 +++++++++++++++++++++++++----
>>>>>>    1 file changed, 25 insertions(+), 4 deletions(-)
>>>>>>
>>>>>> diff --git a/security/integrity/ima/ima_main.c
>>>>>> b/security/integrity/ima/ima_main.c
>>>>>> index 2121a831f38a..e9fa46eedd27 100644
>>>>>> --- a/security/integrity/ima/ima_main.c
>>>>>> +++ b/security/integrity/ima/ima_main.c
>>>>>> @@ -200,10 +200,10 @@ void ima_file_free(struct file *file)
>>>>>>        ima_check_last_writer(iint, inode, file);
>>>>>>    }
>>>>>>    -static int process_measurement(struct ima_namespace *ns,
>>>>>> -                   struct file *file, const struct cred *cred,
>>>>>> -                   u32 secid, char *buf, loff_t size, int mask,
>>>>>> -                   enum ima_hooks func)
>>>>>> +static int _process_measurement(struct ima_namespace *ns,
>>>>> Hm, it's much more common to use double underscores then single
>>>>> underscores to
>>>>>
>>>>> __process_measurement()
>>>>>
>>>>> reads a lot more natural to people perusing kernel code quite often.
>>>>>
>>>>>> +                struct file *file, const struct cred *cred,
>>>>>> +                u32 secid, char *buf, loff_t size, int mask,
>>>>>> +                enum ima_hooks func)
>>>>>>    {
>>>>>>        struct inode *inode = file_inode(file);
>>>>>>        struct integrity_iint_cache *iint = NULL;
>>>>>> @@ -405,6 +405,27 @@ static int process_measurement(struct
>>>>>> ima_namespace *ns,
>>>>>>        return 0;
>>>>>>    }
>>>>>>    +static int process_measurement(struct ima_namespace *ns,
>>>>>> +                   struct file *file, const struct cred *cred,
>>>>>> +                   u32 secid, char *buf, loff_t size, int mask,
>>>>>> +                   enum ima_hooks func)
>>>>>> +{
>>>>>> +    int ret = 0;
>>>>>> +    struct user_namespace *user_ns;
>>>>>> +
>>>>>> +    do {
>>>>>> +        ret = _process_measurement(ns, file, cred, secid, buf,
>>>>>> size, mask, func);
>>>>>> +        if (ret)
>>>>>> +            break;
>>>>>> +        user_ns = ns->user_ns->parent;
>>>>>> +        if (!user_ns)
>>>>>> +            break;
>>>>>> +        ns = user_ns->ima_ns;
>>>>>> +    } while (1);
>>>>> I'd rather write this as:
>>>>>
>>>>>      struct user_namespace *user_ns = ns->user_ns;
>>>>>
>>>>>      while (user_ns) {
>>>>>          ns = user_ns->ima_ns;
>>>>>
>>>>>              ret = __process_measurement(ns, file, cred, secid, buf,
>>>>> size, mask, func);
>>>>>              if (ret)
>>>>>                  break;
>>>>>          user_ns = user_ns->parent;
>>>>>
>>>>>      }
>>>>>
>>>>> because the hierarchy is only an implicit property inherited by ima
>>>>> namespaces from the implementation of user namespaces. In other words,
>>>>> we're only indirectly walking a hierarchy of ima namespaces because
>>>>> we're walking a hierarchy of user namespaces. So the ima ns actually
>>>>> just gives us the entrypoint into the userns hierarchy which the double
>>>>> deref writing it with a while() makes obvious.
>>>> Which brings me to another point.
>>>>
>>>> Technically nothing seems to prevent an ima_ns to survive the
>>>> destruction of its associated userns in ima_ns->user_ns?
>>>>
>>>> One thread does get_ima_ns() and mucks around with it while another one
>>>> does put_user_ns().
>>>>
>>>> Assume it's the last reference to the userns which is now -
>>>> asynchronously - cleaned up from ->work. So at some point you're ending
>>>> with a dangling pointer in ima_ns->user_ns eventually causing a UAF.
>>>>
>>>> If I'm thinking correct than you need to fix this. I can think of two
>>>> ways right now where one of them I'm not sure how well that would work:
>>>> 1. ima_ns takes a reference count to userns at creation. Here you need
>>>>      to make very sure that you're not ending up with reference counting
>>>>      cycles where the two structs keep each other alive.
>>> Right. I am not sure what the trigger would be for ima_ns to release
>>> that one reference.
>>>
>>>
>>>> 2. rcu trickery. That's the one I'm not sure how well that would work
>>>>      where you'd need rcu_read_lock()/rcu_read_unlock() with a
>>>>      get_user_ns() in the middle whenever you're trying to get a ref to
>>>>      the userns from an ima_ns and handle the case where the userns is
>>>>      gone.
>>>>
>>>> Or maybe I'me missing something in the patch series that makes this all
>>>> a non-issue.
>>> I suppose one can always call current_user_ns() to get a pointer to
>>> the current user namespace that the process is accessing the file in
>>> that IMA now reacts to. With the hierarchical processing we are
>>> walking backwards towards init_user_ns. The problem should only exist
>>> if something else frees the current user namespace (or its parents) so
>>> that the hierarchy collapses. Assuming we are always in a process
>>> context then 'current' should protect us, no ?
>>>
>> All existing callers to process_measurements call it at least once with
>> current_cred().
>>
>> The only problem that I see where we are accessing the IMA namespace
>> outside a process context is in 4/16 'ima: Move delayed work queue and
>> variables into ima_namespace' where a delayed work queue is used. I
>> fixed this now by getting an additional reference to the user namesapce
>> before scheduling the delayed work and release it when it ran or when it
>> is canceled (cancel_delayed_work_sync()) but it didn't run.
>>
>  From  the "ima: Move delayed work queue and variables into
> ima_namespace" patch description:
>     Since keys queued up for measurement currently are only relevant in
>     the init_ima_ns, call ima_init_key_queue() only when the init_ima_ns
>     is initialized.
>
> When IMA_QUEUE_EARLY_BOOT_KEYS is not enabled, ima_should_queue_key()
> simply returns false.  Why do the keys workqueue need to be namespaced?
> Is this preparatory for some future IMA namespacing?

06 ima: Move policy related variables into ima_namespace

05 ima: Move IMA's keys queue related variables into ima_namespace

04 ima: Move delayed work queue and variables into ima_namespace

06 requires the ima_namespace parameter to be passed into 
process_buffer_measurement(). The problem was ima_process_queued_keys() 
that needs to pass the namespace but it's probably sufficient to use 
&init_ima_ns there as the ima_namespace parameter, which would allow to 
drop 05 and 04.

   Stefan

>
> thanks,
>
> Mimi
>