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

From: Richard Henderson
Date: Sun Dec 09 2018 - 09:53:51 EST

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}.

It seems like the header comment here, and

> +/*
> + * 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.