On Tue, Nov 03, 2020 at 05:14:43PM +0000, Marc Zyngier wrote:
> We now expose ID_AA64PFR0_EL1.CSV2=1 to guests running on hosts
> that are immune to Spectre-v2, but that don't have this field set,
> most likely because they predate the specification.
>
> However, this prevents the migration of guests that have started on
> a host the doesn't fake this CSV2 setting to one that does, as KVM
> rejects the write to ID_AA64PFR0_EL2 on the grounds that it isn't
> what is already there.
>
> In order to fix this, allow userspace to set this field as long as
> this doesn't result in a promising more than what is already there
> (setting CSV2 to 0 is acceptable, but setting it to 1 when it is
> already set to 0 isn't).
>
> Fixes: e1026237f9067 ("KVM: arm64: Set CSV2 for guests on hardware unaffected by Spectre-v2")
> Reported-by: Peng Liang <liangpeng10@xxxxxxxxxx>
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> ---
> arch/arm64/include/asm/kvm_host.h | 2 ++
> arch/arm64/kvm/arm.c | 16 +++++++++++++
> arch/arm64/kvm/sys_regs.c | 38 +++++++++++++++++++++++++++----
> 3 files changed, 52 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 781d029b8aa8..0cd9f0f75c13 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -118,6 +118,8 @@ struct kvm_arch {
> */
> unsigned long *pmu_filter;
> unsigned int pmuver;
> +
> + u8 pfr0_csv2;
> };
>
> struct kvm_vcpu_fault_info {
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 5750ec34960e..c0ffb019ca8b 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -102,6 +102,20 @@ static int kvm_arm_default_max_vcpus(void)
> return vgic_present ? kvm_vgic_get_max_vcpus() : KVM_MAX_VCPUS;
> }
>
> +static void set_default_csv2(struct kvm *kvm)
> +{
> + /*
> + * The default is to expose CSV2 == 1 if the HW isn't affected.
> + * Although this is a per-CPU feature, we make it global because
> + * asymmetric systems are just a nuisance.
> + *
> + * Userspace can override this as long as it doesn't promise
> + * the impossible.
> + */
> + if (arm64_get_spectre_v2_state() == SPECTRE_UNAFFECTED)
> + kvm->arch.pfr0_csv2 = 1;
> +}
> +
> /**
> * kvm_arch_init_vm - initializes a VM data structure
> * @kvm: pointer to the KVM struct
> @@ -127,6 +141,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> /* The maximum number of VCPUs is limited by the host's GIC model */
> kvm->arch.max_vcpus = kvm_arm_default_max_vcpus();
>
> + set_default_csv2(kvm);
> +
> return ret;
> out_free_stage2_pgd:
> kvm_free_stage2_pgd(&kvm->arch.mmu);
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index fb12d3ef423a..61789027b92b 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1128,9 +1128,8 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
> if (!vcpu_has_sve(vcpu))
> val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
> val &= ~(0xfUL << ID_AA64PFR0_AMU_SHIFT);
> - if (!(val & (0xfUL << ID_AA64PFR0_CSV2_SHIFT)) &&
> - arm64_get_spectre_v2_state() == SPECTRE_UNAFFECTED)
> - val |= (1UL << ID_AA64PFR0_CSV2_SHIFT);
> + val &= ~(0xfUL << ID_AA64PFR0_CSV2_SHIFT);
> + val |= ((u64)vcpu->kvm->arch.pfr0_csv2 << ID_AA64PFR0_CSV2_SHIFT);
> } else if (id == SYS_ID_AA64PFR1_EL1) {
> val &= ~(0xfUL << ID_AA64PFR1_MTE_SHIFT);
> } else if (id == SYS_ID_AA64ISAR1_EL1 && !vcpu_has_ptrauth(vcpu)) {
> @@ -1260,6 +1259,36 @@ static int set_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
> return 0;
> }
>
> +static int set_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,
> + const struct sys_reg_desc *rd,
> + const struct kvm_one_reg *reg, void __user *uaddr)
> +{
> + const u64 id = sys_reg_to_index(rd);
> + int err;
> + u64 val;
> + u8 csv2;
> +
> + err = reg_from_user(&val, uaddr, id);
> + if (err)
> + return err;
> +
> + /*
> + * Allow AA64PFR0_EL1.CSV2 to be set from userspace as long as
> + * it doesn't promise more than what is actually provided (the
> + * guest could otherwise be covered in ectoplasmic residue).
> + */
> + csv2 = FIELD_GET(0xfUL << ID_AA64PFR0_CSV2_SHIFT, val);
cpuid_feature_extract_unsigned_field() instead?
> + if (csv2 > vcpu->kvm->arch.pfr0_csv2)
> + return -EINVAL;
> + vcpu->kvm->arch.pfr0_csv2 = csv2;
> +
> + /* This is what we mean by invariant: you can't change it. */
> + if (val != read_id_reg(vcpu, rd, false))
> + return -EINVAL;
I think it's quite confusing to return -EINVAL in the case that we have
actually updated arch.pfr0_csv2, as it's indistinguishable from the case
when csv2 was invalid and the field wasn't updated.
Will
