Re: [PATCH v6 04/13] arm64/kvm: hide ptrauth from guests

From: Kristina Martsenko
Date: Mon Dec 10 2018 - 15:13:11 EST

On 09/12/2018 14:53, Richard Henderson wrote:
> On 12/7/18 12:39 PM, Kristina Martsenko wrote:
>> From: Mark Rutland <mark.rutland@xxxxxxx>
>> In subsequent patches we're going to expose ptrauth to the host kernel
>> and userspace, but things are a bit trickier for guest kernels. For the
>> time being, let's hide ptrauth from KVM guests.
>> Regardless of how well-behaved the guest kernel is, guest userspace
>> could attempt to use ptrauth instructions, triggering a trap to EL2,
>> resulting in noise from kvm_handle_unknown_ec(). So let's write up a
>> handler for the PAC trap, which silently injects an UNDEF into the
>> guest, as if the feature were really missing.
> Reviewing the long thread that accompanied v5, I thought we were *not* going to
> trap PAuth instructions from the guest.
> In particular, the OS distribution may legitimately be built to include
> hint-space nops. This includes XPACLRI, which is used by the C++ exception
> unwinder and not controlled by SCTLR_EL1.EnI{A,B}.

The plan was to disable trapping, yes. However, after that thread there
was a retrospective change applied to the architecture, such that the
XPACLRI (and XPACD/XPACI) instructions are no longer trapped by
HCR_EL2.API. (The public documentation on this has not been updated
yet.) This means that no HINT-space instructions should trap anymore.
(The guest is expected to not set SCTLR_EL1.EnI{A,B} since
ID_AA64ISAR1_EL1.{APA,API} read as 0.)

> It seems like the header comment here, and
Sorry, which header comment?

>> +/*
>> + * Guest usage of a ptrauth instruction (which the guest EL1 did not turn into
>> + * a NOP).
>> + */
>> +static int kvm_handle_ptrauth(struct kvm_vcpu *vcpu, struct kvm_run *run)
>> +
> here, need updating.

Changed it to "a trapped ptrauth instruction".