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

[Mimi Zohar <zohar@linux.ibm.com>]

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?

thanks,

Mimi