[PATCH 4/7] KVM: arm64: Turn vcpu->arch.pause into a counter

[Marc Zyngier <maz@kernel.org>]

We have situations where we can call kvm_arm_halt_guest() without
holding config_lock, which means that halt and resume can overlap in
funny ways when called from separate contexts, and result in situations
where a vcpu is resumed while other parts of KVM assume it is halted.
Consequences are left to the imagination of the reader.

Fix this sorry situation by turning kvm_vcpu_arch::pause into an
atomic counter, which makes the races described above harmless.

Fixes: 3b92830ad41b2 ("KVM: arm/arm64: implement kvm_arm_[halt,resume]_guest")
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Tested-by: Fuad Tabba < fuad.tabba@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
---
 arch/arm64/include/asm/kvm_host.h |  7 +++----
 arch/arm64/kvm/arm.c              | 21 ++++++++++++---------
 2 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index cd9b9d2462f96..9c068f27f9b8a 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -889,11 +889,10 @@ struct kvm_vcpu_arch {
 	/*
 	 * Don't run the guest (internal implementation need).
 	 *
-	 * Contrary to the flags above, this is set/cleared outside of
-	 * a vcpu context, and thus cannot be mixed with the flags
-	 * themselves (or the flag accesses need to be made atomic).
+	 * Contrary to the flags above, this is updated outside of
+	 * a vcpu context, and thus cannot be mixed with the flags.
 	 */
-	bool pause;
+	atomic_t pause;
 
 	/*
 	 * We maintain more than a single set of debug registers to support
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 9956049e592ab..07bf8b2e683d5 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -565,6 +565,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
 	kvm_arm_pvtime_vcpu_init(&vcpu->arch);
 
 	vcpu->arch.hw_mmu = &vcpu->kvm->arch.mmu;
+	atomic_set(&vcpu->arch.pause, 0);
 
 	/*
 	 * This vCPU may have been created after mpidr_data was initialized.
@@ -839,6 +840,11 @@ int kvm_arch_vcpu_ioctl_set_mpstate(struct kvm_vcpu *vcpu,
 	return ret;
 }
 
+static bool vcpu_can_run(struct kvm_vcpu *vcpu)
+{
+	return !kvm_arm_vcpu_stopped(vcpu) && !atomic_read(&vcpu->arch.pause);
+}
+
 /**
  * kvm_arch_vcpu_runnable - determine if the vcpu can be scheduled
  * @v:		The VCPU pointer
@@ -854,8 +860,7 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *v)
 		      (kvm_timer_should_notify_user(v) ||
 		       kvm_pmu_should_notify_user(v)));
 
-	return ((irq_lines || kvm_vgic_vcpu_pending_irq(v))
-		&& !kvm_arm_vcpu_stopped(v) && !v->arch.pause);
+	return ((irq_lines || kvm_vgic_vcpu_pending_irq(v)) && vcpu_can_run(v));
 }
 
 bool kvm_arch_vcpu_in_kernel(struct kvm_vcpu *vcpu)
@@ -1018,7 +1023,7 @@ void kvm_arm_halt_guest(struct kvm *kvm)
 	struct kvm_vcpu *vcpu;
 
 	kvm_for_each_vcpu(i, vcpu, kvm)
-		vcpu->arch.pause = true;
+		atomic_inc(&vcpu->arch.pause);
 	kvm_make_all_cpus_request(kvm, KVM_REQ_SLEEP);
 }
 
@@ -1028,8 +1033,8 @@ void kvm_arm_resume_guest(struct kvm *kvm)
 	struct kvm_vcpu *vcpu;
 
 	kvm_for_each_vcpu(i, vcpu, kvm) {
-		vcpu->arch.pause = false;
-		__kvm_vcpu_wake_up(vcpu);
+		if (atomic_dec_and_test(&vcpu->arch.pause))
+			__kvm_vcpu_wake_up(vcpu);
 	}
 }
 
@@ -1037,11 +1042,9 @@ static void kvm_vcpu_sleep(struct kvm_vcpu *vcpu)
 {
 	struct rcuwait *wait = kvm_arch_vcpu_get_wait(vcpu);
 
-	rcuwait_wait_event(wait,
-			   (!kvm_arm_vcpu_stopped(vcpu)) && (!vcpu->arch.pause),
-			   TASK_INTERRUPTIBLE);
+	rcuwait_wait_event(wait, vcpu_can_run(vcpu), TASK_INTERRUPTIBLE);
 
-	if (kvm_arm_vcpu_stopped(vcpu) || vcpu->arch.pause) {
+	if (!vcpu_can_run(vcpu)) {
 		/* Awaken to handle a signal, request we sleep again later. */
 		kvm_make_request(KVM_REQ_SLEEP, vcpu);
 	}
-- 
2.47.3