[PATCH v2 14/17] KVM: arm64: Reject host access to protected VM private state
From: Fuad Tabba <fuad.tabba@linux.dev>
Date: 2026-09-07 07:01:44
Also in:
kvmarm, lkml
Subsystem:
arm64 port (aarch64 architecture), kernel virtual machine for arm64 (kvm/arm64), the rest · Maintainers:
Catalin Marinas, Will Deacon, Marc Zyngier, Oliver Upton, Linus Torvalds
A protected vCPU's register and debug state is no longer exposed to
the host. Host ioctls that would reach that state now fail rather
than operate on a copy that is not the guest's:
- KVM_GET_ONE_REG and KVM_SET_ONE_REG return -EPERM once the vCPU has
run: the copy then holds reset values plus what the exit handlers
marshal out. Pre-run access still builds the guest's boot state.
- KVM_ARM_VCPU_INIT returns -EPERM once the vCPU has run: it would
reset the host copy alone and rewrite mp_state, which EL2 reads
only at hyp vCPU creation, so a vCPU the guest powered off would
come back RUNNABLE.
- KVM_SET_VCPU_EVENTS rejects external-abort injection with -EPERM;
SError injection is forwarded and stays permitted.
- KVM_SET_GUEST_DEBUG returns -EPERM: a protected guest's debug state
is hypervisor-owned.
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
arch/arm64/kvm/arm.c | 7 +++++++
arch/arm64/kvm/guest.c | 19 +++++++++++++++++++
2 files changed, 26 insertions(+)
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 5c2587da815ab..37a260b139795 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c@@ -1750,6 +1750,13 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu, bool power_off = false; int ret; + /* + * Re-init would reset the host copy alone and rewrite mp_state + * after EL2 has read it. Pre-run init stays allowed. + */ + if (vcpu_is_protected(vcpu) && vcpu_has_run_once(vcpu)) + return -EPERM; + /* * Treat the power-off vCPU feature as ephemeral. Clear the bit to avoid * reflecting it in the finalized feature set, thus limiting its scope
diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
index ab659795d4b2f..abab580a659d4 100644
--- a/arch/arm64/kvm/guest.c
+++ b/arch/arm64/kvm/guest.c@@ -701,6 +701,10 @@ int kvm_arm_copy_reg_indices(struct kvm_vcpu *vcpu, u64 __user *uindices) int kvm_arm_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg) { + /* Once the vCPU has run, the host copy is not the guest's state. */ + if (vcpu_is_protected(vcpu) && vcpu_has_run_once(vcpu)) + return -EPERM; + /* We currently use nothing arch-specific in upper 32 bits */ if ((reg->id & ~KVM_REG_SIZE_MASK) >> 32 != KVM_REG_ARM64 >> 32) return -EINVAL;
@@ -718,6 +722,10 @@ int kvm_arm_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg) int kvm_arm_set_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg) { + /* Writes build the boot state; once run, EL2 owns the registers. */ + if (vcpu_is_protected(vcpu) && vcpu_has_run_once(vcpu)) + return -EPERM; + /* We currently use nothing arch-specific in upper 32 bits */ if ((reg->id & ~KVM_REG_SIZE_MASK) >> 32 != KVM_REG_ARM64 >> 32) return -EINVAL;
@@ -786,6 +794,13 @@ int __kvm_arm_vcpu_set_events(struct kvm_vcpu *vcpu, u64 esr = events->exception.serror_esr; int ret = 0; + /* + * EL2 injects an external abort only to complete a forwarded abort. + * SError injection is forwarded. + */ + if (vcpu_is_protected(vcpu) && ext_dabt_pending) + return -EPERM; + /* * Immediately commit the pending SEA to the vCPU's architectural * state which is necessary since we do not return a pending SEA
@@ -883,6 +898,10 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu, { trace_kvm_set_guest_debug(vcpu, dbg->control); + /* A protected guest's debug state is not exposed to the host. */ + if (vcpu_is_protected(vcpu)) + return -EPERM; + if (dbg->control & ~KVM_GUESTDBG_VALID_MASK) return -EINVAL;
--
2.39.5