[PATCH 01/13] KVM: arm64: Hide CNTHV_*_EL2 from userspace for nVHE guests

Marc Zyngier
2025-09-29T16:05:05Z

Although we correctly UNDEF any CNTHV_*_EL2 access from the guest
when E2H==0, we still expose these registers to userspace, which
is a bad idea.

Drop the ad-hoc UNDEF injection and switch to a .visibility()
callback which will also hide the register from userspace.

Fixes: 0e45981028550 ("KVM: arm64: timer: Don't adjust the EL2 virtual timer offset")
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
arch/arm64/kvm/sys_regs.c | 26 +++++++++++++-------------
1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index ee8a7033c85bf..9f2f4e0b042e8 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1594,16 +1594,6 @@ static bool access_arch_timer(struct kvm_vcpu *vcpu,
return true;
}

-static bool access_hv_timer(struct kvm_vcpu *vcpu,
- struct sys_reg_params *p,
- const struct sys_reg_desc *r)
-{
- if (!vcpu_el2_e2h_is_set(vcpu))
- return undef_access(vcpu, p, r);
-
- return access_arch_timer(vcpu, p, r);
-}
-
static s64 kvm_arm64_ftr_safe_value(u32 id, const struct arm64_ftr_bits *ftrp,
s64 new, s64 cur)
{
@@ -2831,6 +2821,16 @@ static unsigned int s1pie_el2_visibility(const struct kvm_vcpu *vcpu,
return __el2_visibility(vcpu, rd, s1pie_visibility);
}

+static unsigned int cnthv_visibility(const struct kvm_vcpu *vcpu,
+ const struct sys_reg_desc *rd)
+{
+ if (vcpu_has_nv(vcpu) &&
+ !vcpu_has_feature(vcpu, KVM_ARM_VCPU_HAS_EL2_E2H0))
+ return 0;
+
+ return REG_HIDDEN;
+}
+
static bool access_mdcr(struct kvm_vcpu *vcpu,
struct sys_reg_params *p,
const struct sys_reg_desc *r)
@@ -3691,9 +3691,9 @@ static const struct sys_reg_desc sys_reg_descs[] = {
EL2_REG(CNTHP_CTL_EL2, access_arch_timer, reset_val, 0),
EL2_REG(CNTHP_CVAL_EL2, access_arch_timer, reset_val, 0),

- { SYS_DESC(SYS_CNTHV_TVAL_EL2), access_hv_timer },
- EL2_REG(CNTHV_CTL_EL2, access_hv_timer, reset_val, 0),
- EL2_REG(CNTHV_CVAL_EL2, access_hv_timer, reset_val, 0),
+ { SYS_DESC(SYS_CNTHV_TVAL_EL2), access_arch_timer, .visibility = cnthv_visibility },
+ EL2_REG_FILTERED(CNTHV_CTL_EL2, access_arch_timer, reset_val, 0, cnthv_visibility),
+ EL2_REG_FILTERED(CNTHV_CVAL_EL2, access_arch_timer, reset_val, 0, cnthv_visibility),

{ SYS_DESC(SYS_CNTKCTL_EL12), access_cntkctl_el12 },

--
2.47.3