linux-kernel - Re: [PATCH v3 1/1] psi: Fix uaf issue when psi trigger is destroyed while being polled

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Date:   Wed, 12 Jan 2022 09:43:43 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Eric Biggers <ebiggers@...nel.org>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        cgroups mailinglist <cgroups@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>,
        syzbot <syzbot+cdb5dd11c97cc532efad@...kaller.appspotmail.com>
Subject: Re: [PATCH v3 1/1] psi: Fix uaf issue when psi trigger is destroyed
 while being polled

)

On Wed, Jan 12, 2022 at 6:40 AM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Tue, Jan 11, 2022 at 03:23:09PM -0800, Suren Baghdasaryan wrote:
> > With write operation on psi files replacing old trigger with a new one,
> > the lifetime of its waitqueue is totally arbitrary. Overwriting an
> > existing trigger causes its waitqueue to be freed and pending poll()
> > will stumble on trigger->event_wait which was destroyed.
> > Fix this by disallowing to redefine an existing psi trigger. If a write
> > operation is used on a file descriptor with an already existing psi
> > trigger, the operation will fail with EBUSY error.
> > Also bypass a check for psi_disabled in the psi_trigger_destroy as the
> > flag can be flipped after the trigger is created, leading to a memory
> > leak.
> >
> > Fixes: 0e94682b73bf ("psi: introduce psi monitor")
> > Cc: stable@...r.kernel.org
> > Reported-by: syzbot+cdb5dd11c97cc532efad@...kaller.appspotmail.com
> > Analyzed-by: Eric Biggers <ebiggers@...nel.org>
> > Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
>
> Acked-by: Johannes Weiner <hannes@...xchg.org>

Hmm. kernel test robot notified me of new (which are not really new)
warnings but I don't think this patch specifically introduced them:

kernel/sched/psi.c:1112:21: warning: no previous prototype for
function 'psi_trigger_create' [-Wmissing-prototypes]
   struct psi_trigger *psi_trigger_create(struct psi_group *group,
                       ^
   kernel/sched/psi.c:1112:1: note: declare 'static' if the function
is not intended to be used outside of this translation unit
   struct psi_trigger *psi_trigger_create(struct psi_group *group,
   ^
   static
>> kernel/sched/psi.c:1182:6: warning: no previous prototype for function 'psi_trigger_destroy' [-Wmissing-prototypes]
   void psi_trigger_destroy(struct psi_trigger *t)
        ^
   kernel/sched/psi.c:1182:1: note: declare 'static' if the function
is not intended to be used outside of this translation unit
   void psi_trigger_destroy(struct psi_trigger *t)
   ^
   static
   kernel/sched/psi.c:1249:10: warning: no previous prototype for
function 'psi_trigger_poll' [-Wmissing-prototypes]
   __poll_t psi_trigger_poll(void **trigger_ptr,
            ^
   kernel/sched/psi.c:1249:1: note: declare 'static' if the function
is not intended to be used outside of this translation unit
   __poll_t psi_trigger_poll(void **trigger_ptr,
   ^

This happens with the following config:

CONFIG_CGROUPS=n
CONFIG_PSI=y

With cgroups disabled these functions are defined as non-static but
are not defined in the header
(https://elixir.bootlin.com/linux/latest/source/include/linux/psi.h#L28)
since the only external user cgroup.c is disabled. The cleanest way to
fix these I think is by doing smth like this in psi.c:

struct psi_trigger *_psi_trigger_create(struct psi_group *group, char
*buf, size_t nbytes, enum psi_res res)
{
  // original psi_trigger_create code
}

#ifdef CONFIG_CGROUPS

struct psi_trigger *psi_trigger_create(struct psi_group *group, char
*buf, size_t nbytes, enum psi_res res)
{
    return _psi_trigger_create(group, buf, nbytes, res);
}

#else

static struct psi_trigger *psi_trigger_create(struct psi_group *group,
char *buf, size_t nbytes, enum psi_res res)
{
    return _psi_trigger_create(group, buf, nbytes, res);
}

#endif

Two questions:
1. Is this even worth fixing?
2. If so, I would like to do that as a separate patch (these warnings
are unrelated to the changes in this patch). Would that be ok?
Thanks,
Suren.