Re: [PATCH] kasan: record timestamp of memory allocation/free

[Dmitry Vyukov <dvyukov@google.com>]

On Wed, May 2, 2018 at 12:58 PM, Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
> syzbot is reporting many refcount/use-after-free bugs along with flood of
> memory allocation fault injection messages. Showing timestamp of memory
> allocation/free would help narrowing down kernel messages to examine.
>
> Revive timestamp field which was removed by commit cd11016e5f5212c1
> ("mm, kasan: stackdepot implementation. Enable stackdepot for SLAB").

Hi Tetsuo,

Header real estate is very expensive as it directly contributes to
KASAN memory overhead. We dropped time on purpose to keep header 16
bytes. This doubles it to 32 bytes per heap object. Before we start
putting more stuff  in there, we need to move free-related meta data
into object itself (as it's not used after free) as it was before.

There is also this improvement that also has plans on any spare space in header:
https://bugzilla.kernel.org/show_bug.cgi?id=198437
I would say that rcu_call stack is more important as actual free stack
is usually meaningless for async-freed objects.


> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Andrey Konovalov <adech.fo@gmail.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Konstantin Serebryany <kcc@google.com>
> Cc: Dmitry Chernenkov <dmitryc@google.com>
> ---
>  mm/kasan/kasan.c  | 1 +
>  mm/kasan/kasan.h  | 1 +
>  mm/kasan/report.c | 3 ++-
>  3 files changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
> index 135ce28..a336834 100644
> --- a/mm/kasan/kasan.c
> +++ b/mm/kasan/kasan.c
> @@ -457,6 +457,7 @@ static inline depot_stack_handle_t save_stack(gfp_t flags)
>  static inline void set_track(struct kasan_track *track, gfp_t flags)
>  {
>         track->pid = current->pid;
> +       track->when = jiffies;
>         track->stack = save_stack(flags);
>  }
>
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index c12dcfd..0e4951b 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -77,6 +77,7 @@ struct kasan_global {
>  struct kasan_track {
>         u32 pid;
>         depot_stack_handle_t stack;
> +       unsigned long when;
>  };
>
>  struct kasan_alloc_meta {
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 5c169aa..062c8ae 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -183,7 +183,8 @@ static void kasan_end_report(unsigned long *flags)
>
>  static void print_track(struct kasan_track *track, const char *prefix)
>  {
> -       pr_err("%s by task %u:\n", prefix, track->pid);
> +       pr_err("%s by task %u (%lu jiffies ago):\n", prefix, track->pid,
> +              jiffies - track->when);
>         if (track->stack) {
>                 struct stack_trace trace;
>
> --
> 1.8.3.1
>