linux-kernel - Re: [PATCH v4 4/6] arm64/kvm: enable pointer authentication cpufeature conditionally

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Message-ID: <CADGdYn7kcBXO3YbB+QEfS-Aw4O-G-ssQaapRoXCoG6gD08JjAw@mail.gmail.com>
Date:   Mon, 28 Jan 2019 12:32:52 +0530
From:   Amit Daniel Kachhap <amit.kachhap@....com>
To:     James Morse <james.morse@....com>
Cc:     LAK <linux-arm-kernel@...ts.infradead.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Kristina Martsenko <kristina.martsenko@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        Dave Martin <Dave.Martin@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/6] arm64/kvm: enable pointer authentication
 cpufeature conditionally

Hi James,
On Fri, Jan 4, 2019 at 11:32 PM James Morse <james.morse@....com> wrote:
>
> Hi Amit,
>
> On 18/12/2018 07:56, Amit Daniel Kachhap wrote:
> > According to userspace settings, pointer authentication cpufeature
> > is enabled/disabled from guests.
>
> This reads like the guest is changing something in the host. Isn't this hiding
> the id-register values from the guest?
I dropped this patch altogether in V5 series and now only key
registers are masked
if userspace disables it.

Thanks,
Amit Daniel
>
>
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 6af6c7d..ce6144a 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -1066,6 +1066,15 @@ static u64 read_id_reg(struct sys_reg_desc const *r, bool raz)
> >                       kvm_debug("SVE unsupported for guests, suppressing\n");
> >
> >               val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
> > +     } else if (id == SYS_ID_AA64ISAR1_EL1) {
> > +             const u64 ptrauth_mask = (0xfUL << ID_AA64ISAR1_APA_SHIFT) |
> > +                                      (0xfUL << ID_AA64ISAR1_API_SHIFT) |
> > +                                      (0xfUL << ID_AA64ISAR1_GPA_SHIFT) |
> > +                                      (0xfUL << ID_AA64ISAR1_GPI_SHIFT);
> > +             if (!kvm_arm_vcpu_ptrauth_allowed(vcpu)) {
> > +                     kvm_debug("ptrauth unsupported for guests, suppressing\n");
> > +                     val &= ~ptrauth_mask;
> > +             }
>
> I think this hunk should have been in the previous patch as otherwise its a
> bisection oddity.
>
> Could you merge this hunk with the previous patch, and move the mechanical bits
> that pass vcpu around to a prior preparatory patch.
>
> (I'm still unsure if we need to hide this as a user-controlled policy)
>
>
> Thanks,
>
> James