arm64: kprobe: disable probe of fault prone ptrauth instruction

arm64: kprobe: disable probe of fault prone ptrauth instruction

With the addition of ARMv8.3-FPAC feature, the probe of authenticate
ptrauth instructions (AUT*) may cause ptrauth fault exception in case of
authenticate failure so they cannot be safely single stepped.

Hence the probe of authenticate instructions is disallowed but the
corresponding pac ptrauth instruction (PAC*) is not affected and they can
still be probed. Also AUTH* instructions do not make sense at function
entry points so most realistic probes would be unaffected by this change.

Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
Reviewed-by: Dave Martin <dave.martin@arm.com>
Link: https://lore.kernel.org/r/20200914083656.21428-6-amit.kachhap@arm.com
Signed-off-by: Will Deacon <will@kernel.org>