Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210629161738.936790-1-glider@google.com>
In-Reply-To: <20210629161738.936790-1-glider@google.com>
From:   Souptick Joarder <jrdr.linux@gmail.com>
Date:   Wed, 30 Jun 2021 12:32:43 +0530
Message-ID: <CAFqt6zZ8ZL8WtTg368VJ0WHjXc+YzMuA9D8OBXJ5T9j0ePctQQ@mail.gmail.com>
Subject: Re: [PATCH v2] kfence: skip DMA allocations
To:     Alexander Potapenko <glider@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>, elver@google.com,
        linux-kernel@vger.kernel.org, Linux-MM <linux-mm@kvack.org>,
        stable@vger.kernel.org, Greg KH <gregkh@linuxfoundation.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Jun 29, 2021 at 9:47 PM Alexander Potapenko <glider@google.com> wrote:
>
> Allocation requests with __GFP_DMA/__GFP_DMA32 or
> SLAB_CACHE_DMA/SLAB_CACHE_DMA32 cannot be fulfilled by KFENCE, because
> they must reside in low memory, whereas KFENCE memory pool is located in
> high memory.
>
> Skip such allocations to avoid crashes where DMAable memory is expected.
>
> Fixes: 0ce20dd84089 ("mm: add Kernel Electric-Fence infrastructure")
> 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>
>
> ---
>
> v2:
>  - added parentheses around the GFP clause, as requested by Marco
> ---
>  mm/kfence/core.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index 4d21ac44d5d35..f7ce3d876bc9e 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -760,6 +760,14 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
>         if (size > PAGE_SIZE)
>                 return NULL;
>
> +       /*
> +        * Skip DMA allocations. These must reside in the low memory, which we
> +        * cannot guarantee.
> +        */
> +       if ((flags & (__GFP_DMA | __GFP_DMA32)) ||
> +           (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32)))
> +               return NULL;
> +

I prefer to move this check at the top of the function.
Although it won't make much difference except avoiding atomic operations
in case this condition is true.

>         return kfence_guarded_alloc(s, size, flags);
>  }
>
> --
> 2.32.0.93.g670b81a890-goog
>
>