Re: [PATCH] task_work: kasan: record task_work_add() call stack

[Dmitry Vyukov]

On Mon, Mar 15, 2021 at 10:38 AM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:
>
> On Mon, 2021-03-15 at 07:58 +0100, 'Dmitry Vyukov' via kasan-dev wrote:
> > On Mon, Mar 15, 2021 at 3:00 AM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:
> > >
> > > Why record task_work_add() call stack?
> > > Syzbot reports many use-after-free issues for task_work, see [1].
> > > After see the free stack and the current auxiliary stack, we think
> > > they are useless, we don't know where register the work, this work
> > > may be the free call stack, so that we miss the root cause and
> > > don't solve the use-after-free.
> > >
> > > Add task_work_add() call stack into KASAN auxiliary stack in
> > > order to improve KASAN report. It is useful for programmers
> > > to solve use-after-free issues.
> > >
> > > [1]: https://groups.google.com/g/syzkaller-bugs/search?q=kasan%20use-after-free%20task_work_run
> > >
> > > Signed-off-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> > > Suggested-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > > Cc: Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx>
> > > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > > Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> > > Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Matthias Brugger <matthias.bgg@xxxxxxxxx>
> > > Cc: Jens Axboe <axboe@xxxxxxxxx>
> > > Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> > > ---
> > >  kernel/task_work.c | 3 +++
> > >  mm/kasan/kasan.h   | 2 +-
> > >  2 files changed, 4 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/task_work.c b/kernel/task_work.c
> > > index 9cde961875c0..f255294377da 100644
> > > --- a/kernel/task_work.c
> > > +++ b/kernel/task_work.c
> > > @@ -55,6 +55,9 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
> > >                 break;
> > >         }
> > >
> > > +       /* record the work call stack in order to print it in KASAN reports */
> > > +       kasan_record_aux_stack(work);
> >
> > I think this call should be done _before_ we actually queue the work,
> > because this function may operate on non-current task.
> > Consider, we queue the work, the other task already executes it and
> > triggers use-after-free, now only now we record the stack.
>
> agree, what do you think below change?
>
> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -34,6 +34,9 @@ int task_work_add(struct task_struct *task, struct
> callback_head *work,
>  {
>     struct callback_head *head;
>
> +   /* record the work call stack in order to print it in KASAN reports
> */
> +   kasan_record_aux_stack(work);
> +

This looks good to me.


>     do {
>         head = READ_ONCE(task->task_works);
>         if (unlikely(head == &work_exited))
> @@ -55,9 +58,6 @@ int task_work_add(struct task_struct *task, struct
> callback_head *work,
>         break;
>     }
>
> -   /* record the work call stack in order to print it in KASAN reports
> */
> -   kasan_record_aux_stack(work);
> -
>     return 0;
>  }
>
> > Moreover, I think we can trigger use-after-free here ourselves while
> > recording the aux stack. We queued the work, and the work can cause
> > own free, so it's not necessary live by now.
>
> Sorry, I don't fully know your meaning, do you mean we should add an
> abort when detect use-after-free?

I meant that where we had the kasan_record_aux_stack(work) call in the
first version of the patch, work can be already freed. We must not
access work after queueing it.

> > >         return 0;
> > >  }
> > >
> > > diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> > > index 3436c6bf7c0c..d300fe9415bd 100644
> > > --- a/mm/kasan/kasan.h
> > > +++ b/mm/kasan/kasan.h
> > > @@ -146,7 +146,7 @@ struct kasan_alloc_meta {
> > >         struct kasan_track alloc_track;
> > >  #ifdef CONFIG_KASAN_GENERIC
> > >         /*
> > > -        * call_rcu() call stack is stored into struct kasan_alloc_meta.
> > > +        * Auxiliary stack is stored into struct kasan_alloc_meta.
> > >          * The free stack is stored into struct kasan_free_meta.
> > >          */
> > >         depot_stack_handle_t aux_stack[2];
> > > --
> > > 2.18.0
> >
>
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