linux-kernel - Re: [PATCH mm] kfence: zero guard page after out-of-bounds access

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Message-ID: <CAG_fn=WdpzPxbvzqkpVXjyrUu=GprA2xMBiJdhJqM8cNhABWmw@mail.gmail.com>
Date:   Fri, 12 Mar 2021 14:11:35 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH mm] kfence: zero guard page after out-of-bounds access

On Fri, Mar 12, 2021 at 1:16 PM Marco Elver <elver@...gle.com> wrote:
>
> After an out-of-bounds accesses, zero the guard page before
> re-protecting in kfence_guarded_free(). On one hand this helps make the
> failure mode of subsequent out-of-bounds accesses more deterministic,
> but could also prevent certain information leaks.
>
> Signed-off-by: Marco Elver <elver@...gle.com>
Acked-by: Alexander Potapenko <glider@...gle.com>

> ---
>  mm/kfence/core.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index 3b8ec938470a..f7106f28443d 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -371,6 +371,7 @@ static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool z
>
>         /* Restore page protection if there was an OOB access. */
>         if (meta->unprotected_page) {
> +               memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
>                 kfence_protect(meta->unprotected_page);
>                 meta->unprotected_page = 0;
>         }
> --
> 2.31.0.rc2.261.g7f71774620-goog
>


-- 
Alexander Potapenko
Software Engineer

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