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

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Message-ID: <a2eb07e5-6663-aede-fb75-cc91cf3fffa5@arm.com>
Date:   Mon, 10 Dec 2018 20:12:54 +0000
From:   Kristina Martsenko <kristina.martsenko@....com>
To:     Richard Henderson <richard.henderson@...aro.org>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Adam Wallis <awallis@...eaurora.org>,
        Amit Kachhap <amit.kachhap@....com>,
        Andrew Jones <drjones@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave P Martin <dave.martin@....com>,
        Jacob Bramley <jacob.bramley@....com>,
        Kees Cook <keescook@...omium.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Will Deacon <will.deacon@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 04/13] arm64/kvm: hide ptrauth from guests

On 09/12/2018 14:53, Richard Henderson wrote:
> On 12/7/18 12:39 PM, Kristina Martsenko wrote:
>> From: Mark Rutland <mark.rutland@....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}.

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".

Kristina