[PATCH 2/2] arm64: allow TCR_EL1.TBID0 to be configured

[Catalin Marinas]

On Sat, Nov 21, 2020 at 01:59:03AM -0800, Peter Collingbourne wrote:
> Introduce a Kconfig option that controls whether TCR_EL1.TBID0 is
> set at boot time.
> 
> Setting TCR_EL1.TBID0 increases the number of signature bits used by
> the pointer authentication instructions for instruction addresses by 8,
> which improves the security of pointer authentication, but it also has
> the consequence of changing the operation of the branch instructions
> so that they no longer ignore the top byte of the target address but
> instead fault if they are non-zero. Since this is a change to the
> userspace ABI the option defaults to off.
> 
> Signed-off-by: Peter Collingbourne <pcc at google.com>
> Link: https://linux-review.googlesource.com/id/Ife724ad708142bc475f42e8c1d9609124994bbbd
> ---
> This is more of an RFC. An open question is how to expose this.
> Having it be a build-time flag is probably the simplest option
> but I guess it could also be a boot flag. Since it involves an
> ABI change we may also want a prctl() so that userspace can
> figure out which mode it is in.
> 
> I think we should try to avoid it being a per-task property
> so that we don't need to swap yet another system register on
> task switch.

Having it changed per task at run-time is problematic as this bit may be
cached in the TLB, so it would require a synchronisation across all CPUs
followed by TLBI. It's not even clear to me from the ARM ARM whether
this bit is tagged by ASID, which, if not, would make a per-process
setting impossible.

So this leaves us with a cmdline option. If we are confident that no
software makes use of tagged instruction pointers, we could have it
default on.

Adding Szabolcs on the gcc/glibc side.

-- 
Catalin