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

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 <redacted>
Link: https://linux-review.googlesource.com/id/Ife724ad708142bc475f42e8c1d9609124994bbbd
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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