Re: [PATCH v4 03/11] arm64, kfence: enable KFENCE for ARM64

[Jann Horn <jannh@google.com>]

On Fri, Oct 2, 2020 at 4:19 PM Marco Elver <elver@google.com> wrote:
>
> 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.

Aah, I mixed up high-order pages and compound pages. Sorry for the noise.