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

[Steven Price <steven.price@arm.com>]

On 07/01/2019 12:05, 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      |   9 ++
>  arch/arm64/include/asm/kvm_emulate.h |  14 +++
>  arch/arm64/include/uapi/asm/kvm.h    |   9 ++
>  virt/kvm/arm/psci.c                  | 138 ++++++++++++++++++++++++++-
>  5 files changed, 178 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/include/asm/kvm_emulate.h b/arch/arm/include/asm/kvm_emulate.h
> index 77121b713bef..2255c50debab 100644
> --- a/arch/arm/include/asm/kvm_emulate.h
> +++ b/arch/arm/include/asm/kvm_emulate.h
> @@ -275,6 +275,16 @@ 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..02c93b1d8f6d 100644
> --- a/arch/arm/include/uapi/asm/kvm.h
> +++ b/arch/arm/include/uapi/asm/kvm.h
> @@ -214,6 +214,15 @@ 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_2	KVM_REG_ARM_FW_REG(2)
> +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_MASK	0x3
> +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_NOT_AVAIL	0
> +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_AVAIL	1
> +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNAFFECTED	2
> +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_ENABLED	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 506386a3edde..a44f07f68da4 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -336,6 +336,20 @@ 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..4a19ef199a99 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

I can't help feeling we need more than one bit to deal with all the
possible states. The host can support/not-support the workaround (i.e
the HVC) and the guest can be using/not using the workaround.

In particular I can imagine the following situation:

* Guest starts on a host (host A) without the workaround HVC (so
configures not to use it). Assuming the host doesn't need the workaround
the guest is therefore not vulnerable.

* Migrated to a new host (host B) with the workaround HVC (this is
accepted), the guest is potentially vulnerable.

* Migration back to the original host (host A) is then rejected, even
though the guest isn't using the HVC.

I can see two options here:

* Reject the migration to host B as the guest may be vulnerable after
the migration. I.e. the workaround availability cannot change (either
way) during a migration

* Store an extra bit of information which is whether a particular guest
has the HVC exposed to it. Ideally the HVC handling for the workaround
would also get disabled when running on a host which supports the HVC
but was migrated from a host which doesn't. This prevents problems with
a guest which is e.g. migrated during boot and may do feature detection
after the migration.

Since this is a new ABI it would be good to get the register values
sorted even if we don't have a complete implementation of it.

> +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2	KVM_REG_ARM_FW_REG(2)
> +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_MASK	0x3
> +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_NOT_AVAIL	0
> +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_AVAIL	1
> +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNAFFECTED	2
> +#define KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_ENABLED	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..4c671908ef62 100644
> --- a/virt/kvm/arm/psci.c
> +++ b/virt/kvm/arm/psci.c
> @@ -445,12 +445,18 @@ 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;
>  
>  	return 0;
> @@ -469,6 +475,45 @@ int kvm_arm_get_fw_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  		return 0;
>  	}
>  
> +	if (reg->id == KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1) {
> +		void __user *uaddr = (void __user *)(long)reg->addr;
> +		u64 val = 0;
> +
> +		if (kvm_arm_harden_branch_predictor())
> +			val = KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_AVAIL;
> +
> +		if (copy_to_user(uaddr, &val, KVM_REG_SIZE(reg->id)))
> +			return -EFAULT;
> +
> +		return 0;
> +	}
> +
> +	if (reg->id == KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2) {
> +		void __user *uaddr = (void __user *)(long)reg->addr;
> +		u64 val = KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_NOT_AVAIL;
> +
> +		switch (kvm_arm_have_ssbd()) {
> +		case KVM_SSBD_FORCE_DISABLE:
> +		case KVM_SSBD_UNKNOWN:
> +			break;
> +		case KVM_SSBD_KERNEL:
> +			val |= KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_AVAIL;
> +			break;
> +		case KVM_SSBD_FORCE_ENABLE:
> +		case KVM_SSBD_MITIGATED:
> +			val |= KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNAFFECTED;
> +			break;
> +		}
> +
> +		if (kvm_arm_get_vcpu_workaround_2_flag(vcpu))
> +			val |= KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_ENABLED;
> +
> +		if (copy_to_user(uaddr, &val, KVM_REG_SIZE(reg->id)))
> +			return -EFAULT;
> +
> +		return 0;
> +	}
> +
>  	return -EINVAL;
>  }
>  
> @@ -499,5 +544,94 @@ int kvm_arm_set_fw_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  		}
>  	}
>  
> +	if (reg->id == KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1) {
> +		void __user *uaddr = (void __user *)(long)reg->addr;
> +		u64 val;
> +
> +		if (copy_from_user(&val, uaddr, KVM_REG_SIZE(reg->id)))
> +			return -EFAULT;
> +
> +		/* Make sure we support WORKAROUND_1 if userland asks for it. */
> +		if ((val & KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_AVAIL) &&
> +		    !kvm_arm_harden_branch_predictor())
> +			return -EINVAL;
> +
> +		/* Any other bit is reserved. */
> +		if (val & ~KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_AVAIL)
> +			return -EINVAL;
> +
> +		return 0;
> +	}
> +
> +	if (reg->id == KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2) {
> +		void __user *uaddr = (void __user *)(long)reg->addr;
> +		unsigned int wa_state;
> +		bool wa_flag;
> +		u64 val;
> +
> +		if (copy_from_user(&val, uaddr, KVM_REG_SIZE(reg->id)))
> +			return -EFAULT;
> +
> +		/* Reject any unknown bits. */
> +		if (val & ~(KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_MASK|
> +			    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_ENABLED))
> +			return -EINVAL;
> +
> +		/*
> +		 * The value passed from userland has to be compatible with
> +		 * our own workaround status. We also have to consider the
> +		 * requested per-VCPU state for some combinations:
> +		 * --------------+-----------+-----------------+---------------
> +		 * \ user value  |           |                 |
> +		 *  ------------ | SSBD_NONE |   SSBD_KERNEL   |  SSBD_ALWAYS
> +		 *  this kernel \|           |                 |
> +		 * --------------+-----------+-----------------+---------------
> +		 * UNKNOWN       |     OK    |   -EINVAL       |   -EINVAL
> +		 * FORCE_DISABLE |           |                 |
> +		 * --------------+-----------+-----------------+---------------
> +		 * KERNEL        |     OK    | copy VCPU state | set VCPU state
> +		 * --------------+-----------+-----------------+---------------
> +		 * FORCE_ENABLE  |     OK    |      OK         |      OK
> +		 * MITIGATED     |           |                 |
> +		 * --------------+-----------+-----------------+---------------
> +		 */
> +
> +		wa_state = val & KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_MASK;
> +		switch (wa_state) {
> +		case  KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_NOT_AVAIL:
> +			/* We can always support no mitigation (1st column). */
> +			return 0;
> +		case KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_AVAIL:
> +		case KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNAFFECTED:
> +			break;
> +		default:
> +			return -EINVAL;
> +		}
> +
> +		switch (kvm_arm_have_ssbd()) {
> +		case KVM_SSBD_UNKNOWN:
> +		case KVM_SSBD_FORCE_DISABLE:
> +		default:
> +			/* ... but some mitigation was requested (1st line). */
> +			return -EINVAL;
> +		case KVM_SSBD_FORCE_ENABLE:
> +		case KVM_SSBD_MITIGATED:
> +			/* Always-on is always compatible (3rd line). */
> +			return 0;
> +		case KVM_SSBD_KERNEL:		/* 2nd line */
> +			wa_flag = val;
> +			wa_flag |= KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_MASK;
> +
> +			/* Force on when always-on is requested. */
> +			if (wa_state == KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNAFFECTED)
> +				wa_flag = true;
> +			break;
> +		}
> +
> +		kvm_arm_set_vcpu_workaround_2_flag(vcpu, wa_flag);

Since this line is only reached in the KVM_SSBD_KERNEL case I think it
should be moved up. I'd personally find the code easier to follow if the
default/UNKNOWN/FORCE_DISABLE case is the one that drops out and all the
others have a "return 0". It took me a while to be sure that wa_flag
wasn't used uninitialised here!

Steve

> +
> +		return 0;
> +	}
> +
>  	return -EINVAL;
>  }
>