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    SubjectRe: [PATCH v4 03/14] KVM: arm64: Kill 32-bit vCPUs on systems with mismatched EL0 support
    On Friday 27 Nov 2020 at 17:14:11 (+0000), Marc Zyngier wrote:
    > On 2020-11-27 11:53, Will Deacon wrote:
    > > On Fri, Nov 27, 2020 at 10:26:47AM +0000, Marc Zyngier wrote:
    > > > On 2020-11-24 15:50, Will Deacon wrote:
    > > > > If a vCPU is caught running 32-bit code on a system with mismatched
    > > > > support at EL0, then we should kill it.
    > > > >
    > > > > Acked-by: Marc Zyngier <maz@kernel.org>
    > > > > Signed-off-by: Will Deacon <will@kernel.org>
    > > > > ---
    > > > > arch/arm64/kvm/arm.c | 11 ++++++++++-
    > > > > 1 file changed, 10 insertions(+), 1 deletion(-)
    > > > >
    > > > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
    > > > > index 5750ec34960e..d322ac0f4a8e 100644
    > > > > --- a/arch/arm64/kvm/arm.c
    > > > > +++ b/arch/arm64/kvm/arm.c
    > > > > @@ -633,6 +633,15 @@ static void check_vcpu_requests(struct kvm_vcpu
    > > > > *vcpu)
    > > > > }
    > > > > }
    > > > >
    > > > > +static bool vcpu_mode_is_bad_32bit(struct kvm_vcpu *vcpu)
    > > > > +{
    > > > > + if (likely(!vcpu_mode_is_32bit(vcpu)))
    > > > > + return false;
    > > > > +
    > > > > + return !system_supports_32bit_el0() ||
    > > > > + static_branch_unlikely(&arm64_mismatched_32bit_el0);
    > > > > +}
    > > > > +
    > > > > /**
    > > > > * kvm_arch_vcpu_ioctl_run - the main VCPU run function to execute
    > > > > guest code
    > > > > * @vcpu: The VCPU pointer
    > > > > @@ -816,7 +825,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
    > > > > * with the asymmetric AArch32 case), return to userspace with
    > > > > * a fatal error.
    > > > > */
    > > > > - if (!system_supports_32bit_el0() && vcpu_mode_is_32bit(vcpu)) {
    > > > > + if (vcpu_mode_is_bad_32bit(vcpu)) {
    > > > > /*
    > > > > * As we have caught the guest red-handed, decide that
    > > > > * it isn't fit for purpose anymore by making the vcpu
    > > >
    > > > Given the new definition of system_supports_32bit_el0() in the
    > > > previous
    > > > patch,
    > > > why do we need this patch at all?
    > >
    > > I think the check is still needed, as this is an unusual case where we
    > > want to reject the mismatched system. For example, imagine
    > > 'arm64_mismatched_32bit_el0' is true and we're on a mismatched system:
    > > in
    > > this case system_supports_32bit_el0() will return 'true' because we
    > > allow 32-bit applications to run, we support the 32-bit personality etc.
    > >
    > > However, we still want to terminate 32-bit vCPUs if we spot them in this
    > > situation, so we have to check for:
    > >
    > > !system_supports_32bit_el0() ||
    > > static_branch_unlikely(&arm64_mismatched_32bit_el0)
    > >
    > > so that we only allow 32-bit vCPUs when all of the physical CPUs support
    > > it at EL0.
    > >
    > > I could make this clearer either by adding a comment, or avoiding
    > > system_supports_32bit_el0() entirely here and just checking the
    > > sanitised SYS_ID_AA64PFR0_EL1 register directly instead.
    > >
    > > What do you prefer?
    >
    > Yeah, the sanitized read feels better, if only because that is
    > what we are going to read in all the valid cases, unfortunately.
    > read_sanitised_ftr_reg() is sadly not designed to be called on
    > a fast path, meaning that 32bit guests will do a bsearch() on
    > the ID-regs every time they exit...
    >
    > I guess we will have to evaluate how much we loose with this.

    Could we use the trick we have for arm64_ftr_reg_ctrel0 to speed this
    up?

    Thanks,
    Quentin

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