linux-kernel - Re: [PATCH v3] KVM: arm64: Use BTI for nvhe

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Date:   Thu, 6 Jul 2023 15:23:32 +0000
From:   Mostafa Saleh <smostafa@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     oliver.upton@...ux.dev, Sudeep Holla <sudeep.holla@....com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, tabba@...gle.com, qperret@...gle.com,
        will@...nel.org, catalin.marinas@....com, yuzenghui@...wei.com,
        suzuki.poulose@....com, james.morse@....com, bgardon@...gle.com,
        gshan@...hat.com
Subject: Re: [PATCH v3] KVM: arm64: Use BTI for nvhe

On Thu, Jul 06, 2023 at 03:27:36PM +0100, Marc Zyngier wrote:
> Hi Mostafa,
> 
> On Thu, 06 Jul 2023 13:49:04 +0100,
> Mostafa Saleh <smostafa@...gle.com> wrote:
> > 
> > Hi Marc and Oliver,
> > 
> > I was double checking that nothing else was missed.
> > 
> > I found there is another problem for hw that has BTI and is affected
> > by specterv3a.
> > 
> > "br'' instructions are generated at runtime for the vector
> > table(__bp_harden_hyp_vecs).  These branches would land on vectors
> > in __kvm_hyp_vector at offset 8.
> > 
> > As all the macros are defined with valid_vect/invalid_vect, it is
> > sufficient to add "bti j" there at the correct offset.
> >
> > I am not sure if such hardware exists. I tested this with a stubbed
> > "has_spectre_v3a" which confirms the issue and the fix.
> 
> Thanks for the heads up.
> 
> Fortunately, there is no such HW as far as I can tell. Only Cortex-A57
> and A72 are affected by this (and the only two CPUs for which we
> engage the mitigation), and they are way too old to know about BTI.
> 
> > Please let me know if this fix suitable, I can include it with the other fix in
> > "[PATCH] KVM: arm64: Add missing BTI instruction in kvm_host_psci_cpu_entry"
> > 
> > diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
> > index 8f3f93fa119e..175c030379e3 100644
> > --- a/arch/arm64/kvm/hyp/hyp-entry.S
> > +++ b/arch/arm64/kvm/hyp/hyp-entry.S
> > @@ -154,6 +154,12 @@ SYM_CODE_END(\label)
> >   esb
> >   stp x0, x1, [sp, #-16]!
> >  662:
> > + /*
> > + * Specter vectors __bp_harden_hyp_vecs generate br instructions at runtime
> > + * that jump at offset 8 at __kvm_hyp_vector.
> > + * As hyp .text is guarded section, it needs bti j.
> > + */
> > + bti j
> >   b \target
> > 
> >  check_preamble_length 661b, 662b
> > @@ -165,6 +171,8 @@ check_preamble_length 661b, 662b
> >   nop
> >   stp x0, x1, [sp, #-16]!
> >  662:
> > + /* Check valid_vect */
> > + bti j
> >   b \target
> > 
> >  check_preamble_length 661b, 662b
> 
> This looks correct to me.
> 
> If you can respin you initial patch (with maybe a slightly more
> generic subject) so that Oliver can pick it up as part of the next
> batch of fixes, that'd be great.

Thanks a lot, I just sent v2.
[PATCH v2] KVM: arm64: Add missing BTI instructions

> Thanks,
> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.