Re: [PATCH v6 3/9] arm64, kfence: enable KFENCE for ARM64

[Marco Elver <elver@google.com>]

On Fri, 30 Oct 2020 at 16:47, Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Thu, Oct 29, 2020 at 02:16:43PM +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>
> > ---
> > v5:
> > * Move generic page allocation code to core.c [suggested by Jann Horn].
> > * Remove comment about HAVE_ARCH_KFENCE_STATIC_POOL, since we no longer
> >   support static pools.
> > * Force page granularity for the linear map [suggested by Mark Rutland].
> > ---
> >  arch/arm64/Kconfig              |  1 +
> >  arch/arm64/include/asm/kfence.h | 19 +++++++++++++++++++
> >  arch/arm64/mm/fault.c           |  4 ++++
> >  arch/arm64/mm/mmu.c             |  7 ++++++-
> >  4 files changed, 30 insertions(+), 1 deletion(-)
> >  create mode 100644 arch/arm64/include/asm/kfence.h
> >
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index f858c352f72a..2f8b32dddd8b 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -135,6 +135,7 @@ config ARM64
> >       select HAVE_ARCH_JUMP_LABEL_RELATIVE
> >       select HAVE_ARCH_KASAN if !(ARM64_16K_PAGES && ARM64_VA_BITS_48)
> >       select HAVE_ARCH_KASAN_SW_TAGS if HAVE_ARCH_KASAN
> > +     select HAVE_ARCH_KFENCE if (!ARM64_16K_PAGES && !ARM64_64K_PAGES)
>
> Why does this depend on the page size?
>
> If this is functional, but has a larger overhead on 16K or 64K, I'd
> suggest removing the dependency, and just updating the Kconfig help text
> to explain that.

Good point, I don't think anything is requiring us to force 4K pages.
Let's remove it.

Thanks,
-- Marco

> Otherwise, this patch looks fine to me.
>
> Thanks,
> Mark.