[PATCH v6 3/3] arm64: pac: Optimize kernel entry/exit key installation code paths

[James Morse]

Hi Peter,

On 12/02/2021 05:01, Peter Collingbourne wrote:
> On Tue, Jan 26, 2021 at 5:09 AM Will Deacon <will at kernel.org> wrote:
>>
>> On Tue, Dec 29, 2020 at 10:59:15PM -0800, Peter Collingbourne wrote:
>>> The kernel does not use any keys besides IA so we don't need to
>>> install IB/DA/DB/GA on kernel exit if we arrange to install them
>>> on task switch instead, which we can expect to happen an order of
>>> magnitude less often.
>>>
>>> Furthermore we can avoid installing the user IA in the case where the
>>> user task has IA disabled and just leave the kernel IA installed. This
>>> also lets us avoid needing to install IA on kernel entry.
>>
>> I've got to be honest, this makes me nervous in case there is a way for
>> userspace to recover the kernel key even though EnIA is clear. Currently,
>> EnIA doesn't affect XPAC* and PACGA instructions, and the architecture

> For GA I would expect it to be controlled by a hypothetical EnGA, not
> by EnIA (and I'm a bit surprised that there isn't an EnGA;

PACGA is undefined if the CPU doesn't implement PAC, whereas PACIASP is a NOP if the CPU
doesn't implement PAC.

I think the reason from the SCTLR_ELx controls is to make unaware systems transform the
instructions that were hints back into hints. (e.g. the AddPACIA psuedo code). This is
needed on mismatched big-little systems, otherwise processes can't be migrated between them.

For the non-hint instructions, user-space needs to test the hwcap/id-register-emulation to
know it can use these instructions, and the compiler shouldn't output them unconditionally.


> doesn't it
> mean that a userspace program running under an unaware kernel or
> hypervisor may sign things using the GA from potentially another
> hypervisor guest?)

The hypervisor controls all this with HCR_EL2.API, which also traps PACGA et al.
For the hypervisor its all or nothing.
If the hypervisor is emulating a machine without PAC, it can emulate an undefined
exception regardless of whether the CPU supports PAC or not.

Does this match your reading?


Thanks,

James