Re: [PATCH v3 1/2] kfence: move the size check to the beginning of __kfence_alloc()

[Marco Elver <elver@google.com>]

On Wed, 30 Jun 2021 at 15:53, Alexander Potapenko <glider@google.com> wrote:
>
> Check the allocation size before toggling kfence_allocation_gate.
> This way allocations that can't be served by KFENCE will not result in
> waiting for another CONFIG_KFENCE_SAMPLE_INTERVAL without allocating
> anything.
>
> Suggested-by: Marco Elver <elver@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Marco Elver <elver@google.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: stable@vger.kernel.org # 5.12+
> Signed-off-by: Alexander Potapenko <glider@google.com>

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


> ---
>  mm/kfence/core.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index 4d21ac44d5d35..33bb20d91bf6a 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -733,6 +733,13 @@ void kfence_shutdown_cache(struct kmem_cache *s)
>
>  void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
>  {
> +       /*
> +        * Perform size check before switching kfence_allocation_gate, so that
> +        * we don't disable KFENCE without making an allocation.
> +        */
> +       if (size > PAGE_SIZE)
> +               return NULL;
> +
>         /*
>          * allocation_gate only needs to become non-zero, so it doesn't make
>          * sense to continue writing to it and pay the associated contention
> @@ -757,9 +764,6 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
>         if (!READ_ONCE(kfence_enabled))
>                 return NULL;
>
> -       if (size > PAGE_SIZE)
> -               return NULL;
> -
>         return kfence_guarded_alloc(s, size, flags);
>  }
>
> --
> 2.32.0.93.g670b81a890-goog
>