[01/10] KVM: x86: Don't take kvm->lock when iterating over vCPUs in suspend notifier

Commit Message

Sean Christopherson Jan. 18, 2025, 12:55 a.m. UTC

When queueing vCPU PVCLOCK updates in response to SUSPEND or HIBERNATE,
don't take kvm->lock as doing so can trigger a largely theoretical
deadlock, it is perfectly safe to iterate over the xarray of vCPUs without
holding kvm->lock, and kvm->lock doesn't protect kvm_set_guest_paused() in
any way (pv_time.active and pvclock_set_guest_stopped_request are
protected by vcpu->mutex, not kvm->lock).

Reported-by: [e-mail]
Closes: https://lore.kernel.org/all/[e-mail]
Fixes: 7d62874f69d7 ("kvm: x86: implement KVM PM-notifier")
Signed-off-by: Sean Christopherson <[e-mail]>
---
 arch/x86/kvm/x86.c | 2 --
 1 file changed, 2 deletions(-)

Comments

Paul Durrant Jan. 21, 2025, 4:01 p.m. UTC | #1

On 18/01/2025 00:55, Sean Christopherson wrote:
> When queueing vCPU PVCLOCK updates in response to SUSPEND or HIBERNATE,
> don't take kvm->lock as doing so can trigger a largely theoretical
> deadlock, it is perfectly safe to iterate over the xarray of vCPUs without
> holding kvm->lock, and kvm->lock doesn't protect kvm_set_guest_paused() in
> any way (pv_time.active and pvclock_set_guest_stopped_request are
> protected by vcpu->mutex, not kvm->lock).
> 
> Reported-by: [e-mail]
> Closes: https://lore.kernel.org/all/[e-mail]
> Fixes: 7d62874f69d7 ("kvm: x86: implement KVM PM-notifier")
> Signed-off-by: Sean Christopherson <[e-mail]>

Reviewed-by: Paul Durrant <[e-mail]>

Patch

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b2d9a16fd4d3..26e18c9b0375 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6907,7 +6907,6 @@  static int kvm_arch_suspend_notifier(struct kvm *kvm)
 	unsigned long i;
 	int ret = 0;
 
-	mutex_lock(&kvm->lock);
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		if (!vcpu->arch.pv_time.active)
 			continue;
@@ -6919,7 +6918,6 @@  static int kvm_arch_suspend_notifier(struct kvm *kvm)
 			break;
 		}
 	}
-	mutex_unlock(&kvm->lock);
 
 	return ret ? NOTIFY_BAD : NOTIFY_DONE;
 }