Re: [PATCH 00/17] ARMv8.3 pointer authentication support

[Will Deacon <will.deacon@arm.com>]

On Fri, Oct 05, 2018 at 09:47:37AM +0100, Kristina Martsenko wrote:
> 1) Key support
> 
> This series enables the use of instructions using APIAKey, which is
> initialised and maintained per-process (shared by all threads). GCC
> currently only makes use of APIAKey.
> 
> This series does not add support for APIBKey, APDAKey, APDBKey, nor
> APGAKey. HINT-space instructions using these keys will currently execute
> as NOPs. Support for these keys can be added as users appear.
> 
> Note that while we expose the cpuid register (ID_AA64ISAR1_EL1) to
> userspace, it only contains one feature for address authentication
> (API/APA), so it cannot be used by userspace to tell which keys the
> kernel supports. For this the kernel exposes HWCAP bits, one per key
> (currently only APIAKey), which must be checked instead.

Given that the architecture doesn't provide an identification mechanism
for the case where only one of the keys is available, I would much prefer
that we expose both of the keys to userspace. Is the only downside of
that a possible exception entry overhead if the kernel wants to use pointer
authentication as well?

Having an initial implementation where the B key operations act as NOPs
isn't ideal if we want to support future users -- chances are they'll
be put off because deployed kernels don't give them whatever security
guarantees they require. It's a bit of a chicken-and-egg problem, so
unless we have good reasons to keep the B key hidden, I think we should
be exposing it from the start.

Will