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

Stefan Berger
2021-12-16T02:55:46Z

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
>