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SubjectRe: [PATCH v4 03/14] KVM: arm64: Kill 32-bit vCPUs on systems with mismatched EL0 support
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.

Thanks,

M.
--
Jazz is not dead. It just smells funny...

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