[PATCH v2 14/17] KVM: arm64: Reject host access to protected VM private state

[Fuad Tabba <fuad.tabba@linux.dev>]

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