Re: [PATCH mm] kasan: avoid resetting aux_lock

[Marco Elver <elver@google.com>]

On Tue, 9 Jan 2024 at 23:12, <andrey.konovalov@linux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@gmail.com>
>
> With commit 63b85ac56a64 ("kasan: stop leaking stack trace handles"),
> KASAN zeroes out alloc meta when an object is freed. The zeroed out data
> purposefully includes alloc and auxiliary stack traces but also
> accidentally includes aux_lock.
>
> As aux_lock is only initialized for each object slot during slab
> creation, when the freed slot is reallocated, saving auxiliary stack
> traces for the new object leads to lockdep reports when taking the
> zeroed out aux_lock.
>
> Arguably, we could reinitialize aux_lock when the object is reallocated,
> but a simpler solution is to avoid zeroing out aux_lock when an object
> gets freed.
>
> Reported-by: Paul E. McKenney <paulmck@kernel.org>
> Closes: https://lore.kernel.org/linux-next/5cc0f83c-e1d6-45c5-be89-9b86746fe731@paulmck-laptop/
> Fixes: 63b85ac56a64 ("kasan: stop leaking stack trace handles")
> Signed-off-by: Andrey Konovalov <andreyknvl@gmail.com>

Reviewed-by: Marco Elver <elver@google.com>

> ---
>  mm/kasan/generic.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> index 24c13dfb1e94..df6627f62402 100644
> --- a/mm/kasan/generic.c
> +++ b/mm/kasan/generic.c
> @@ -487,6 +487,7 @@ void kasan_init_object_meta(struct kmem_cache *cache, const void *object)
>                 __memset(alloc_meta, 0, sizeof(*alloc_meta));
>
>                 /*
> +                * Prepare the lock for saving auxiliary stack traces.
>                  * Temporarily disable KASAN bug reporting to allow instrumented
>                  * raw_spin_lock_init to access aux_lock, which resides inside
>                  * of a redzone.
> @@ -510,8 +511,13 @@ static void release_alloc_meta(struct kasan_alloc_meta *meta)
>         stack_depot_put(meta->aux_stack[0]);
>         stack_depot_put(meta->aux_stack[1]);
>
> -       /* Zero out alloc meta to mark it as invalid. */
> -       __memset(meta, 0, sizeof(*meta));
> +       /*
> +        * Zero out alloc meta to mark it as invalid but keep aux_lock
> +        * initialized to avoid having to reinitialize it when another object
> +        * is allocated in the same slot.
> +        */
> +       __memset(&meta->alloc_track, 0, sizeof(meta->alloc_track));
> +       __memset(meta->aux_stack, 0, sizeof(meta->aux_stack));
>  }
>
>  static void release_free_meta(const void *object, struct kasan_free_meta *meta)
> --
> 2.25.1
>