Messages in this thread |  | | Date | Fri, 27 Nov 2020 17:14:11 +0000 | From | Marc Zyngier <> | Subject | Re: [PATCH v4 03/14] KVM: arm64: Kill 32-bit vCPUs on systems with mismatched EL0 support |
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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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