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KVM: arm64: Allow setting of ID_AA64PFR0_EL1.CSV2 from userspace

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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: e1026237f9 ("KVM: arm64: Set CSV2 for guests on hardware unaffected by Spectre-v2")
Reported-by: Peng Liang <liangpeng10@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20201110141308.451654-2-maz@kernel.org
This commit is contained in:
Marc Zyngier 2020-11-10 14:13:06 +00:00
parent 4f6b838c37
commit 23711a5e66
3 changed files with 56 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
arch/arm64/include/asm/kvm_host.h
View File

@ -118,6 +118,8 @@ struct kvm_arch {
*/
unsigned long *pmu_filter;
unsigned int pmuver;
u8 pfr0_csv2;
};
struct kvm_vcpu_fault_info {

16
arch/arm64/kvm/arm.c
View File

@ -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);

42
arch/arm64/kvm/sys_regs.c
View File

@ -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)) {
@ -1213,6 +1212,40 @@ static unsigned int sve_visibility(const struct kvm_vcpu *vcpu,
return REG_HIDDEN;
}
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 = cpuid_feature_extract_unsigned_field(val, ID_AA64PFR0_CSV2_SHIFT);
if (csv2 > 1 ||
(csv2 && arm64_get_spectre_v2_state() != SPECTRE_UNAFFECTED))
return -EINVAL;
/* We can only differ with CSV2, and anything else is an error */
val ^= read_id_reg(vcpu, rd, false);
val &= ~(0xFUL << ID_AA64PFR0_CSV2_SHIFT);
if (val)
return -EINVAL;
vcpu->kvm->arch.pfr0_csv2 = csv2;
return 0;
}
/*
* cpufeature ID register user accessors
*
@ -1472,7 +1505,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
/* AArch64 ID registers */
/* CRm=4 */
ID_SANITISED(ID_AA64PFR0_EL1),
{ SYS_DESC(SYS_ID_AA64PFR0_EL1), .access = access_id_reg,
.get_user = get_id_reg, .set_user = set_id_aa64pfr0_el1, },
ID_SANITISED(ID_AA64PFR1_EL1),
ID_UNALLOCATED(4,2),
ID_UNALLOCATED(4,3),