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

[Walter Wu]

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);
+
    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?

> 
> >         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
>