KVM: arm64: Avoid vcpu->mutex v. kvm->lock inversion in CPU_ON

KVM/arm64 had the lock ordering backwards on vcpu->mutex and kvm->lock
from the very beginning. One such example is the way vCPU resets are
handled: the kvm->lock is acquired while handling a guest CPU_ON PSCI
call.

Add a dedicated lock to serialize writes to kvm_vcpu_arch::{mp_state,
reset_state}. Promote all accessors of mp_state to {READ,WRITE}_ONCE()
as readers do not acquire the mp_state_lock. While at it, plug yet
another race by taking the mp_state_lock in the KVM_SET_MP_STATE ioctl
handler.

As changes to MP state are now guarded with a dedicated lock, drop the
kvm->lock acquisition from the PSCI CPU_ON path. Similarly, move the
reader of reset_state outside of the kvm->lock and instead protect it
with the mp_state_lock. Note that writes to reset_state::reset have been
demoted to regular stores as both readers and writers acquire the
mp_state_lock.

While the kvm->lock inversion still exists in kvm_reset_vcpu(), at least
now PSCI CPU_ON no longer depends on it for serializing vCPU reset.

Cc: stable@vger.kernel.org
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230327164747.2466958-2-oliver.upton@linux.dev

1 files changed, 22 insertions, 9 deletions

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 3bd732eaf087..647798da8c41 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -326,6 +326,8 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
 {
 	int err;
 
+	spin_lock_init(&vcpu->arch.mp_state_lock);
+
 	/* Force users to call KVM_ARM_VCPU_INIT */
 	vcpu->arch.target = -1;
 	bitmap_zero(vcpu->arch.features, KVM_VCPU_MAX_FEATURES);
@@ -443,34 +445,41 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 	vcpu->cpu = -1;
 }
 
-void kvm_arm_vcpu_power_off(struct kvm_vcpu *vcpu)
+static void __kvm_arm_vcpu_power_off(struct kvm_vcpu *vcpu)
 {
-	vcpu->arch.mp_state.mp_state = KVM_MP_STATE_STOPPED;
+	WRITE_ONCE(vcpu->arch.mp_state.mp_state, KVM_MP_STATE_STOPPED);
 	kvm_make_request(KVM_REQ_SLEEP, vcpu);
 	kvm_vcpu_kick(vcpu);
 }
 
+void kvm_arm_vcpu_power_off(struct kvm_vcpu *vcpu)
+{
+	spin_lock(&vcpu->arch.mp_state_lock);
+	__kvm_arm_vcpu_power_off(vcpu);
+	spin_unlock(&vcpu->arch.mp_state_lock);
+}
+
 bool kvm_arm_vcpu_stopped(struct kvm_vcpu *vcpu)
 {
-	return vcpu->arch.mp_state.mp_state == KVM_MP_STATE_STOPPED;
+	return READ_ONCE(vcpu->arch.mp_state.mp_state) == KVM_MP_STATE_STOPPED;
 }
 
 static void kvm_arm_vcpu_suspend(struct kvm_vcpu *vcpu)
 {
-	vcpu->arch.mp_state.mp_state = KVM_MP_STATE_SUSPENDED;
+	WRITE_ONCE(vcpu->arch.mp_state.mp_state, KVM_MP_STATE_SUSPENDED);
 	kvm_make_request(KVM_REQ_SUSPEND, vcpu);
 	kvm_vcpu_kick(vcpu);
 }
 
 static bool kvm_arm_vcpu_suspended(struct kvm_vcpu *vcpu)
 {
-	return vcpu->arch.mp_state.mp_state == KVM_MP_STATE_SUSPENDED;
+	return READ_ONCE(vcpu->arch.mp_state.mp_state) == KVM_MP_STATE_SUSPENDED;
 }
 
 int kvm_arch_vcpu_ioctl_get_mpstate(struct kvm_vcpu *vcpu,
 				    struct kvm_mp_state *mp_state)
 {
-	*mp_state = vcpu->arch.mp_state;
+	*mp_state = READ_ONCE(vcpu->arch.mp_state);
 
 	return 0;
 }
@@ -480,12 +489,14 @@ int kvm_arch_vcpu_ioctl_set_mpstate(struct kvm_vcpu *vcpu,
 {
 	int ret = 0;
 
+	spin_lock(&vcpu->arch.mp_state_lock);
+
 	switch (mp_state->mp_state) {
 	case KVM_MP_STATE_RUNNABLE:
-		vcpu->arch.mp_state = *mp_state;
+		WRITE_ONCE(vcpu->arch.mp_state, *mp_state);
 		break;
 	case KVM_MP_STATE_STOPPED:
-		kvm_arm_vcpu_power_off(vcpu);
+		__kvm_arm_vcpu_power_off(vcpu);
 		break;
 	case KVM_MP_STATE_SUSPENDED:
 		kvm_arm_vcpu_suspend(vcpu);
@@ -494,6 +505,8 @@ int kvm_arch_vcpu_ioctl_set_mpstate(struct kvm_vcpu *vcpu,
 		ret = -EINVAL;
 	}
 
+	spin_unlock(&vcpu->arch.mp_state_lock);
+
 	return ret;
 }
 
@@ -1213,7 +1226,7 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu,
 	if (test_bit(KVM_ARM_VCPU_POWER_OFF, vcpu->arch.features))
 		kvm_arm_vcpu_power_off(vcpu);
 	else
-		vcpu->arch.mp_state.mp_state = KVM_MP_STATE_RUNNABLE;
+		WRITE_ONCE(vcpu->arch.mp_state.mp_state, KVM_MP_STATE_RUNNABLE);
 
 	return 0;
 }