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, 5 insertions, 4 deletions

diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
index 49a3257dec46..9e023546bde0 100644
--- a/arch/arm64/kvm/reset.c
+++ b/arch/arm64/kvm/reset.c
@@ -264,15 +264,16 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 
 	mutex_lock(&vcpu->kvm->lock);
 	ret = kvm_set_vm_width(vcpu);
-	if (!ret) {
-		reset_state = vcpu->arch.reset_state;
-		WRITE_ONCE(vcpu->arch.reset_state.reset, false);
-	}
 	mutex_unlock(&vcpu->kvm->lock);
 
 	if (ret)
 		return ret;
 
+	spin_lock(&vcpu->arch.mp_state_lock);
+	reset_state = vcpu->arch.reset_state;
+	vcpu->arch.reset_state.reset = false;
+	spin_unlock(&vcpu->arch.mp_state_lock);
+
 	/* Reset PMU outside of the non-preemptible section */
 	kvm_pmu_vcpu_reset(vcpu);