Re: [PATCH v4 1/2] KVM: arm/arm64: Add save/restore support for firmware workaround state

[Andre Przywara <andre.przywara@arm.com>]

On Thu, 21 Mar 2019 13:54:07 +0100
Auger Eric <eric.auger@redhat.com> wrote:

Hi Eric,

sorry, forgot to hit "Send" this morning ;-)

> On 3/1/19 12:43 AM, Andre Przywara wrote:
> > KVM implements the firmware interface for mitigating cache speculation
> > vulnerabilities. Guests may use this interface to ensure mitigation is
> > active.
> > If we want to migrate such a guest to a host with a different support
> > level for those workarounds, migration might need to fail, to ensure that
> > critical guests don't loose their protection.
> > 
> > Introduce a way for userland to save and restore the workarounds state.
> > On restoring we do checks that make sure we don't downgrade our
> > mitigation level.
> > 
> > Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> > ---
> >  arch/arm/include/asm/kvm_emulate.h   |  10 +++
> >  arch/arm/include/uapi/asm/kvm.h      |  10 +++
> >  arch/arm64/include/asm/kvm_emulate.h |  14 +++
> >  arch/arm64/include/uapi/asm/kvm.h    |   9 ++
> >  virt/kvm/arm/psci.c                  | 128 +++++++++++++++++++++++----
> >  5 files changed, 155 insertions(+), 16 deletions(-)
> > 
> > diff --git a/arch/arm/include/asm/kvm_emulate.h b/arch/arm/include/asm/kvm_emulate.h
> > index 8927cae7c966..663a02d7e6f4 100644
> > --- a/arch/arm/include/asm/kvm_emulate.h
> > +++ b/arch/arm/include/asm/kvm_emulate.h
> > @@ -283,6 +283,16 @@ static inline unsigned long kvm_vcpu_get_mpidr_aff(struct kvm_vcpu *vcpu)
> >  	return vcpu_cp15(vcpu, c0_MPIDR) & MPIDR_HWID_BITMASK;
> >  }
> >  
> > +static inline bool kvm_arm_get_vcpu_workaround_2_flag(struct kvm_vcpu *vcpu)
> > +{
> > +	return false;
> > +}
> > +
> > +static inline void kvm_arm_set_vcpu_workaround_2_flag(struct kvm_vcpu *vcpu,
> > +						      bool flag)
> > +{
> > +}
> > +
> >  static inline void kvm_vcpu_set_be(struct kvm_vcpu *vcpu)
> >  {
> >  	*vcpu_cpsr(vcpu) |= PSR_E_BIT;
> > diff --git a/arch/arm/include/uapi/asm/kvm.h b/arch/arm/include/uapi/asm/kvm.h
> > index 4602464ebdfb..ba4d2afe65e3 100644
> > --- a/arch/arm/include/uapi/asm/kvm.h
> > +++ b/arch/arm/include/uapi/asm/kvm.h
> > @@ -214,6 +214,16 @@ struct kvm_vcpu_events {
> >  #define KVM_REG_ARM_FW_REG(r)		(KVM_REG_ARM | KVM_REG_SIZE_U64 | \
> >  					 KVM_REG_ARM_FW | ((r) & 0xffff))
> >  #define KVM_REG_ARM_PSCI_VERSION	KVM_REG_ARM_FW_REG(0)
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1	KVM_REG_ARM_FW_REG(1)
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_NOT_AVAIL	0
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_AVAIL	1
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_UNAFFECTED	2
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2	KVM_REG_ARM_FW_REG(2)
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_NOT_AVAIL	0
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNKNOWN	1  
> Would be worth adding a comment saying that values are chosen so that
> higher values mean better protection. Otherwise it looks strange
> NOT_AVAIL/AVAIL/UNAFFECTED values are not the same for both workarounds.	
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_AVAIL	2
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNAFFECTED	3
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_ENABLED	(1U << 4)  
> 
> >  
> >  /* Device Control API: ARM VGIC */
> >  #define KVM_DEV_ARM_VGIC_GRP_ADDR	0
> > diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> > index d3842791e1c4..c00c17c9adb6 100644
> > --- a/arch/arm64/include/asm/kvm_emulate.h
> > +++ b/arch/arm64/include/asm/kvm_emulate.h
> > @@ -348,6 +348,20 @@ static inline unsigned long kvm_vcpu_get_mpidr_aff(struct kvm_vcpu *vcpu)
> >  	return vcpu_read_sys_reg(vcpu, MPIDR_EL1) & MPIDR_HWID_BITMASK;
> >  }
> >  
> > +static inline bool kvm_arm_get_vcpu_workaround_2_flag(struct kvm_vcpu *vcpu)
> > +{
> > +	return vcpu->arch.workaround_flags & VCPU_WORKAROUND_2_FLAG;
> > +}
> > +
> > +static inline void kvm_arm_set_vcpu_workaround_2_flag(struct kvm_vcpu *vcpu,
> > +						      bool flag)
> > +{
> > +	if (flag)
> > +		vcpu->arch.workaround_flags |= VCPU_WORKAROUND_2_FLAG;
> > +	else
> > +		vcpu->arch.workaround_flags &= ~VCPU_WORKAROUND_2_FLAG;
> > +}
> > +
> >  static inline void kvm_vcpu_set_be(struct kvm_vcpu *vcpu)
> >  {
> >  	if (vcpu_mode_is_32bit(vcpu)) {
> > diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> > index 97c3478ee6e7..367e96fe654e 100644
> > --- a/arch/arm64/include/uapi/asm/kvm.h
> > +++ b/arch/arm64/include/uapi/asm/kvm.h
> > @@ -225,6 +225,15 @@ struct kvm_vcpu_events {
> >  #define KVM_REG_ARM_FW_REG(r)		(KVM_REG_ARM64 | KVM_REG_SIZE_U64 | \
> >  					 KVM_REG_ARM_FW | ((r) & 0xffff))
> >  #define KVM_REG_ARM_PSCI_VERSION	KVM_REG_ARM_FW_REG(0)
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1	KVM_REG_ARM_FW_REG(1)
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_NOT_AVAIL	0
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_AVAIL	1
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2	KVM_REG_ARM_FW_REG(2)
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_NOT_AVAIL	0
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNKNOWN	1
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_AVAIL	2
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNAFFECTED	3
> > +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_ENABLED     (1U << 4)
> >  
> >  /* Device Control API: ARM VGIC */
> >  #define KVM_DEV_ARM_VGIC_GRP_ADDR	0
> > diff --git a/virt/kvm/arm/psci.c b/virt/kvm/arm/psci.c
> > index 9b73d3ad918a..e65664c09b12 100644
> > --- a/virt/kvm/arm/psci.c
> > +++ b/virt/kvm/arm/psci.c
> > @@ -445,42 +445,97 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
> >  
> >  int kvm_arm_get_fw_num_regs(struct kvm_vcpu *vcpu)
> >  {
> > -	return 1;		/* PSCI version */
> > +	return 3;		/* PSCI version and two workaround registers */
> >  }
> >  
> >  int kvm_arm_copy_fw_reg_indices(struct kvm_vcpu *vcpu, u64 __user *uindices)
> >  {
> > -	if (put_user(KVM_REG_ARM_PSCI_VERSION, uindices))
> > +	if (put_user(KVM_REG_ARM_PSCI_VERSION, uindices++))
> > +		return -EFAULT;
> > +
> > +	if (put_user(KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1, uindices++))
> > +		return -EFAULT;
> > +
> > +	if (put_user(KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2, uindices++))
> >  		return -EFAULT;  
> Wouldn't it make sense to have a const array somewhere listing the FW
> regs and putting KVM_REG_ARM_FW_REG[i]? Also kvm_arm_get_fw_num_regs
> could return the ARRAY_SIZE.

Decided to cheekily leaving this exercise to the next user of the firmware register interface - or to yet another version of this series ;-)

> vcpu arg is never used actually (not related to this patch).

I think there is some sense in allowing per-VCPU values of firmware registers, we just don't use it at the moment, since we rely on homogeneous CPUs for most operations. But that's just a property of the currently existing firmware registers.

> >  
> >  	return 0;
> >  }
> >  
> > +#define KVM_REG_FEATURE_LEVEL_WIDTH	4
> > +#define KVM_REG_FEATURE_LEVEL_MASK	(BIT(KVM_REG_FEATURE_LEVEL_WIDTH) - 1)
> > +
> > +/*
> > + * Convert the workaround level into an easy-to-compare number, where higher
> > + * values mean better protection.
> > + */
> > +static int get_kernel_wa_level(u64 regid)
> > +{
> > +	switch (regid) {
> > +	case KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1:
> > +		if (kvm_arm_harden_branch_predictor())
> > +			return KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_AVAIL;
> > +		else
> > +			return KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_NOT_AVAIL;
> > +	case KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2:
> > +		switch (kvm_arm_have_ssbd()) {
> > +		case KVM_SSBD_FORCE_DISABLE:
> > +			return KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_NOT_AVAIL;
> > +		case KVM_SSBD_KERNEL:
> > +			return KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_AVAIL;
> > +		case KVM_SSBD_FORCE_ENABLE:
> > +		case KVM_SSBD_MITIGATED:
> > +			return KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNAFFECTED;
> > +		case KVM_SSBD_UNKNOWN:
> > +		default:
> > +			return KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNKNOWN;
> > +		}
> > +	}
> > +
> > +	return 0;  
> I would rather return -EINVAL although the function is not called for
> any invalid reg.

Good point.

> > +}
> > +
> >  int kvm_arm_get_fw_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
> >  {
> > -	if (reg->id == KVM_REG_ARM_PSCI_VERSION) {
> > -		void __user *uaddr = (void __user *)(long)reg->addr;
> > -		u64 val;
> > +	void __user *uaddr = (void __user *)(long)reg->addr;
> > +	u64 val;
> >  
> > +	switch (reg->id) {
> > +	case KVM_REG_ARM_PSCI_VERSION:
> >  		val = kvm_psci_version(vcpu, vcpu->kvm);
> > -		if (copy_to_user(uaddr, &val, KVM_REG_SIZE(reg->id)))
> > -			return -EFAULT;
> > -
> > -		return 0;
> > +		break;
> > +	case KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1:
> > +		val = get_kernel_wa_level(reg->id) & KVM_REG_FEATURE_LEVEL_MASK;  
> Can get_kernel_wa_level return something outside of
> KVM_REG_FEATURE_LEVEL_MASK?

No, not at the moment. But I find it better to keep the mask, as this goes out to userland, so we want to make sure this is within the documented range.

Cheers,
Andre.


> > +		break;
> > +	case KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2:
> > +		val = get_kernel_wa_level(reg->id) & KVM_REG_FEATURE_LEVEL_MASK;  
> same here
> > +		if (kvm_arm_have_ssbd() == KVM_SSBD_KERNEL &&> +		    kvm_arm_get_vcpu_workaround_2_flag(vcpu))  
> nit: if (val == KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_AVAIL &&
> kvm_arm_get_vcpu_workaround_2_flag(vcpu)).
> > +			val |= KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_ENABLED;
> > +		break;
> > +	default:
> > +		return -ENOENT;
> >  	}
> >  
> > -	return -EINVAL;
> > +	if (copy_to_user(uaddr, &val, KVM_REG_SIZE(reg->id)))
> > +		return -EFAULT;
> > +
> > +	return 0;
> >  }
> >  
> >  int kvm_arm_set_fw_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
> >  {
> > -	if (reg->id == KVM_REG_ARM_PSCI_VERSION) {
> > -		void __user *uaddr = (void __user *)(long)reg->addr;
> > -		bool wants_02;
> > -		u64 val;
> > +	void __user *uaddr = (void __user *)(long)reg->addr;
> > +	u64 val;
> > +	int wa_level;
> > +
> > +	if (copy_from_user(&val, uaddr, KVM_REG_SIZE(reg->id)))
> > +		return -EFAULT;
> >  
> > -		if (copy_from_user(&val, uaddr, KVM_REG_SIZE(reg->id)))
> > -			return -EFAULT;
> > +	switch (reg->id) {
> > +	case KVM_REG_ARM_PSCI_VERSION:
> > +	{
> > +		bool wants_02;
> >  
> >  		wants_02 = test_bit(KVM_ARM_VCPU_PSCI_0_2, vcpu->arch.features);
> >  
> > @@ -497,6 +552,47 @@ int kvm_arm_set_fw_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
> >  			vcpu->kvm->arch.psci_version = val;
> >  			return 0;
> >  		}
> > +		break;
> > +	}
> > +
> > +	case KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1:
> > +		if (val & ~KVM_REG_FEATURE_LEVEL_MASK)
> > +			return -EINVAL;
> > +
> > +		wa_level = val & KVM_REG_FEATURE_LEVEL_MASK;  
> not needed
> > +
> > +		/* For now we only accept the very same workaround level. */
> > +		if (get_kernel_wa_level(reg->id) != wa_level)
> > +			return -EINVAL;
> > +
> > +		return 0;
> > +
> > +	case KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2:
> > +		if (val & ~(KVM_REG_FEATURE_LEVEL_MASK |
> > +			    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_ENABLED))> +			return -EINVAL;
> > +
> > +		wa_level = val & KVM_REG_FEATURE_LEVEL_MASK;
> > +
> > +		if (get_kernel_wa_level(reg->id) < wa_level)
> > +			return -EINVAL;
> > +  
> worth a comment?
> > +		if (kvm_arm_have_ssbd() != KVM_SSBD_KERNEL)
> > +			return 0;
> > +
> > +		switch (wa_level) {
> > +		case KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_AVAIL:
> > +			kvm_arm_set_vcpu_workaround_2_flag(vcpu,
> > +			    val & KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_ENABLED);
> > +			break;
> > +		case KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNAFFECTED:
> > +			kvm_arm_set_vcpu_workaround_2_flag(vcpu, true);  
> Looks strange to me we enable the flag when unaffected.
> 
> > +			break;
> > +		}
> > +
> > +		return 0;
> > +	default:
> > +		return -ENOENT;
> >  	}
> >  
> >  	return -EINVAL;
> >   
> 
> Thanks
> 
> Eric