Re: [PATCH] KVM: arm64: Prevent mixed-width VM creation

On Thu, May 20, 2021 at 01:58:55PM +0100, Marc Zyngier wrote:

On Thu, 20 May 2021 13:44:34 +0100,
Mark Rutland [off-list ref] wrote:

quoted

On Thu, May 20, 2021 at 01:22:53PM +0100, Marc Zyngier wrote:

quoted

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.

Sorry; I should have been clearer. I had assumed that this was trying to
rely on a difference across vcpus implicitly providing an equivalent of
the removed check for the KVM_ARM_VCPU_EL1_32BIT cap. I guess from the
below that was not the case. :)

Thanks,
Mark.

But of course...

quoted

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...

quoted

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.

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

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