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From:   Marco Elver <elver@google.com>
Date:   Fri, 2 Oct 2020 16:18:48 +0200
Message-ID: <CANpmjNMcdM2MSL5J6ewChovxZbe-rKncU4LekQiXwKoVY0xDnQ@mail.gmail.com>
Subject: Re: [PATCH v4 03/11] arm64, kfence: enable KFENCE for ARM64
To:     Jann Horn <jannh@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
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On Fri, 2 Oct 2020 at 08:48, Jann Horn <jannh@google.com> wrote:
>
> On Tue, Sep 29, 2020 at 3:38 PM Marco Elver <elver@google.com> wrote:
> > Add architecture specific implementation details for KFENCE and enable
> > KFENCE for the arm64 architecture. In particular, this implements the
> > required interface in <asm/kfence.h>. Currently, the arm64 version does
> > not yet use a statically allocated memory pool, at the cost of a pointer
> > load for each is_kfence_address().
> [...]
> > diff --git a/arch/arm64/include/asm/kfence.h b/arch/arm64/include/asm/kfence.h
> [...]
> > +static inline bool arch_kfence_initialize_pool(void)
> > +{
> > +       const unsigned int num_pages = ilog2(roundup_pow_of_two(KFENCE_POOL_SIZE / PAGE_SIZE));
> > +       struct page *pages = alloc_pages(GFP_KERNEL, num_pages);
> > +
> > +       if (!pages)
> > +               return false;
> > +
> > +       __kfence_pool = page_address(pages);
> > +       return true;
> > +}
>
> If you're going to do "virt_to_page(meta->addr)->slab_cache = cache;"
> on these pages in kfence_guarded_alloc(), and pass them into kfree(),
> you'd better mark these pages as non-compound - something like
> alloc_pages_exact() or split_page() may help. Otherwise, I think when
> SLUB's kfree() does virt_to_head_page() right at the start, that will
> return a pointer to the first page of the entire __kfence_pool, and
> then when it loads page->slab_cache, it gets some random cache and
> stuff blows up. Kinda surprising that you haven't run into that during
> your testing, maybe I'm missing something...

I added a WARN_ON() check in kfence_initialize_pool() to check if our
pages are compound or not; they are not.

In slub.c, __GFP_COMP is passed to alloc_pages(), which causes them to
have a compound head I believe.

> Also, this kinda feels like it should be the "generic" version of
> arch_kfence_initialize_pool() and live in mm/kfence/core.c ?

Done for v5.

Thanks,
-- Marco