Re: [PATCH v3 03/10] arm64, kfence: enable KFENCE for ARM64

[Alexander Potapenko]

On Mon, Sep 21, 2020 at 4:31 PM Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Mon, Sep 21, 2020 at 03:26:04PM +0200, Marco Elver 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().
> >
> > Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Co-developed-by: Alexander Potapenko <glider@xxxxxxxxxx>
> > Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> > ---
> > For ARM64, we would like to solicit feedback on what the best option is
> > to obtain a constant address for __kfence_pool. One option is to declare
> > a memory range in the memory layout to be dedicated to KFENCE (like is
> > done for KASAN), however, it is unclear if this is the best available
> > option. We would like to avoid touching the memory layout.
>
> Sorry for the delay on this.

NP, thanks for looking!

> Given that the pool is relatively small (i.e. when compared with our virtual
> address space), dedicating an area of virtual space sounds like it makes
> the most sense here. How early do you need it to be available?

Yes, having a dedicated address sounds good.
We're inserting kfence_init() into start_kernel() after timekeeping_init().
So way after mm_init(), if that matters.

> An alternative approach would be to patch in the address at runtime, with
> something like a static key to swizzle off the direct __kfence_pool load
> once we're up and running.

IIUC there's no such thing as address patching in the kernel at the
moment, at least static keys work differently?
I am not sure how much we need to randomize this address range (we
don't on x86 anyway).

> Will
>
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Alexander Potapenko
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