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

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

[Walter Wu]

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@mediatek.com>
Suggested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Oleg Nesterov <oleg@redhat.com>
---
 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);
+
 	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

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

[Dmitry Vyukov]

On Mon, Mar 15, 2021 at 3:00 AM Walter Wu <walter-zh.wu@mediatek.com> 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@mediatek.com>
> Suggested-by: Dmitry Vyukov <dvyukov@google.com>
> Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Andrey Konovalov <andreyknvl@google.com>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Matthias Brugger <matthias.bgg@gmail.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Oleg Nesterov <oleg@redhat.com>
> ---
>  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.
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.

>         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