Re: [PATCH v7 3/9] arm64, kfence: enable KFENCE for ARM64

[Marco Elver <elver@google.com>]

On Wed, 4 Nov 2020 at 14:06, Mark Rutland <mark.rutland@arm.com> wrote:
> On Tue, Nov 03, 2020 at 06:58:35PM +0100, 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>.
> >
> > KFENCE requires that attributes for pages from its memory pool can
> > individually be set. Therefore, force the entire linear map to be mapped
> > at page granularity. Doing so may result in extra memory allocated for
> > page tables in case rodata=full is not set; however, currently
> > CONFIG_RODATA_FULL_DEFAULT_ENABLED=y is the default, and the common case
> > is therefore not affected by this change.
> >
> > Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> > Co-developed-by: Alexander Potapenko <glider@google.com>
> > Signed-off-by: Alexander Potapenko <glider@google.com>
> > Signed-off-by: Marco Elver <elver@google.com>
>
> Thanks for dilligently handling all the review feedback. This looks good
> to me now, so FWIW:
>
> Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Thank you!

> There is one thing that I thing we should improve as a subsequent
> cleanup, but I don't think that should block this as-is.
>
> > +#define KFENCE_SKIP_ARCH_FAULT_HANDLER "el1_sync"
>
> IIUC, the core kfence code is using this to figure out where to trace
> from when there's a fault taken on an access to a protected page.

Correct.

> It would be better if the arch code passed the exception's pt_regs into
> the kfence fault handler, and the kfence began the trace began from
> there. That would also allow for dumping the exception registers which
> can help with debugging (e.g. figuring out how the address was derived
> when it's calculated from multiple source registers). That would also be
> a bit more robust to changes in an architectures' exception handling
> code.

Good idea, thanks. I guess there's no reason to not want to always
skip to instruction_pointer(regs)?
In which case I can prepare a patch to make this change. If this
should go into a v8, please let me know. But it'd be easier as a
subsequent patch as you say, given it'll be easier to review and these
patches are in -mm now.

Thanks,
-- Marco