On Thu, 20 May 2021 13:44:34 +0100, Mark Rutland mark.rutland@arm.com wrote:
On Thu, May 20, 2021 at 01:22:53PM +0100, Marc Zyngier wrote:
It looks like we have tolerated creating mixed-width VMs since... forever. However, that was never the intention, and we'd rather not have to support that pointless complexity.
Forbid such a setup by making sure all the vcpus have the same register width.
Reported-by: Steven Price steven.price@arm.com Signed-off-by: Marc Zyngier maz@kernel.org Cc: stable@vger.kernel.org
arch/arm64/kvm/reset.c | 28 ++++++++++++++++++++++++---- 1 file changed, 24 insertions(+), 4 deletions(-)
diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c index 956cdc240148..1cf308be6ef3 100644 --- a/arch/arm64/kvm/reset.c +++ b/arch/arm64/kvm/reset.c @@ -166,6 +166,25 @@ static int kvm_vcpu_enable_ptrauth(struct kvm_vcpu *vcpu) return 0; } +static bool vcpu_allowed_register_width(struct kvm_vcpu *vcpu) +{
- struct kvm_vcpu *tmp;
- int i;
- /* Check that the vcpus are either all 32bit or all 64bit */
- kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
bool w;w = test_bit(KVM_ARM_VCPU_EL1_32BIT, tmp->arch.features);w ^= test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features);if (w)return false;- }
I think this is wrong for a single-cpu VM. In that case, the loop will have a single iteration, and tmp == vcpu, so w must be 0 regardless of the value of arch.features.
I don't immediately see what is wrong with a single-cpu VM. 'w' will be zero indeed, and we'll return that this is allowed. After all, each VM starts by being a single-CPU VM.
But of course...
IIUC that doesn't prevent KVM_ARM_VCPU_EL1_32BIT being set when we don't have the ARM64_HAS_32BIT_EL1 cap, unless that's checked elsewhere?
... I mistakenly removed the check against ARM64_HAS_32BIT_EL1...
How about something like:
| static bool vcpu_allowed_register_width(struct kvm_vcpu *vcpu) | { | bool is_32bit = vcpu_features_32bit(vcpu); | struct kvm_vcpu *tmp; | int i; | | if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1) && is_32bit) | return false; | | kvm_for_each_vcpu(i, tmp, vcpu->kvm) { | if (is_32bit != vcpu_features_32bit(tmp)) | return false; | } | | return true; | }
... with a helper in <asm/kvm_emulate.h> like:
| static bool vcpu_features_32bit(struct kvm_vcpu *vcpu) | { | return test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features); | }
... or
| static inline bool vcpu_has_feature(struct kvm_vcpu *vcpu, int feature) | { | return test_bit(feature, vcpu->arch.features); | }
... so that we can avoid the line splitting required by the length of the test_bit() expression?
Yup, looks OK to me (with a preference for the latter).
Thanks,
M.