[PATCH 2/2] arm64: Configure kernel's PTR_AUTH key when it is built with PTR_AUTH.

[Catalin Marinas]

On Mon, Dec 07, 2020 at 03:07:07PM -0800, Peter Collingbourne wrote:
> On Mon, Dec 7, 2020 at 2:46 PM Daniel Kiss <daniel.kiss at arm.com> wrote:
> > If the kernel is not compiled with CONFIG_ARM64_PTR_AUTH_KERNEL,
> > then the kernel does not need a key and kernel's key could be disabled.
> >
> > Signed-off-by: Daniel Kiss <daniel.kiss at arm.com>
> > ---
> >  arch/arm64/include/asm/asm_pointer_auth.h | 68 ++++++++++++++++-------
> >  arch/arm64/include/asm/processor.h        |  2 +
> >  arch/arm64/kernel/asm-offsets.c           |  4 ++
> >  3 files changed, 55 insertions(+), 19 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/asm_pointer_auth.h b/arch/arm64/include/asm/asm_pointer_auth.h
> > index 52dead2a8640..af3d16027e8f 100644
> > --- a/arch/arm64/include/asm/asm_pointer_auth.h
> > +++ b/arch/arm64/include/asm/asm_pointer_auth.h
> > @@ -14,6 +14,12 @@
> >   * thread.keys_user.ap*.
> >   */
> >         .macro ptrauth_keys_install_user tsk, tmp1, tmp2, tmp3
> > +#ifndef CONFIG_ARM64_PTR_AUTH_KERNEL
> > +       /* Reenable A key */
> > +       mrs     \tmp1, sctlr_el1
> > +       orr     \tmp1, \tmp1, SCTLR_ELx_ENIA
> > +       msr     sctlr_el1, \tmp1
> > +#endif
> 
> We should avoid an unconditional MSR on exit like this as it is
> expensive (for my PR_PAC_SET_ENABLED_KEYS series I measured the cost
> of entry/exit MSR as 43.7ns on Cortex-A75 and 33.0ns on Apple M1). In
> that series I take care not to touch SCTLR_EL1 unless necessary.
> Likewise for the MSRs on entry below.

I think that's how Daniel attempted the first (internal) version of
these patches. In theory you don't need to touch SCTLR_ELx_EN* at all as
long as the kernel does not use any PAC instructions. However, I was
a bit concerned about this and thought it's safer if, when
!CONFIG_ARM64_PTR_AUTH_KERNEL, the EnIA bit is cleared while in the
kernel.

If we can guarantee that the compiler does not generate any PAC
instructions (it may assume they are no-ops) and vendor modules don't
have such instructions either, we may be able to relax this.

-- 
Catalin