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Subject: Re: [PATCH v6 04/13] arm64/kvm: hide ptrauth from guests
To:     Kristina Martsenko <kristina.martsenko@arm.com>,
        linux-arm-kernel@lists.infradead.org
Cc:     Adam Wallis <awallis@codeaurora.org>,
        Amit Kachhap <amit.kachhap@arm.com>,
        Andrew Jones <drjones@redhat.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christoffer Dall <christoffer.dall@arm.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave P Martin <dave.martin@arm.com>,
        Jacob Bramley <jacob.bramley@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org
References: <20181207183931.4285-1-kristina.martsenko@arm.com>
 <20181207183931.4285-5-kristina.martsenko@arm.com>
From:   Richard Henderson <richard.henderson@linaro.org>
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On 12/7/18 12:39 PM, Kristina Martsenko wrote:
> From: Mark Rutland <mark.rutland@arm.com>
> 
> 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.


r~