diff mbox series

KVM: arm64: Prevent mixed-width VM creation

Message ID 20210520122253.171545-1-maz@kernel.org
State Superseded
Headers show
Series KVM: arm64: Prevent mixed-width VM creation | expand

Commit Message

Marc Zyngier May 20, 2021, 12:22 p.m. UTC
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(-)

Comments

Mark Rutland May 20, 2021, 12:44 p.m. UTC | #1
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.

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?

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?

Thanks,
Mark.

> +
> +	return true;
> +}
> +
>  /**
>   * kvm_reset_vcpu - sets core registers and sys_regs to reset value
>   * @vcpu: The VCPU pointer
> @@ -217,13 +236,14 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
>  		}
>  	}
>  
> +	if (!vcpu_allowed_register_width(vcpu)) {
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +
>  	switch (vcpu->arch.target) {
>  	default:
>  		if (test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features)) {
> -			if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1)) {
> -				ret = -EINVAL;
> -				goto out;
> -			}
>  			pstate = VCPU_RESET_PSTATE_SVC;
>  		} else {
>  			pstate = VCPU_RESET_PSTATE_EL1;
> -- 
> 2.30.2
> 
> _______________________________________________
> kvmarm mailing list
> kvmarm@lists.cs.columbia.edu
> https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
Marc Zyngier May 20, 2021, 12:58 p.m. UTC | #2
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.
Mark Rutland May 20, 2021, 2:06 p.m. UTC | #3
On Thu, May 20, 2021 at 01:58:55PM +0100, Marc Zyngier wrote:
> 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.

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...
> 
> > 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.
> 
> -- 
> Without deviation from the norm, progress is not possible.
diff mbox series

Patch

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;
+	}
+
+	return true;
+}
+
 /**
  * kvm_reset_vcpu - sets core registers and sys_regs to reset value
  * @vcpu: The VCPU pointer
@@ -217,13 +236,14 @@  int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 		}
 	}
 
+	if (!vcpu_allowed_register_width(vcpu)) {
+		ret = -EINVAL;
+		goto out;
+	}
+
 	switch (vcpu->arch.target) {
 	default:
 		if (test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features)) {
-			if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1)) {
-				ret = -EINVAL;
-				goto out;
-			}
 			pstate = VCPU_RESET_PSTATE_SVC;
 		} else {
 			pstate = VCPU_RESET_PSTATE_EL1;