Patch "KVM: Disable irq while unregistering user notifier" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

KVM: Disable irq while unregistering user notifier

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-disable-irq-while-unregistering-user-notifier.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ignacio Alvarado <[email protected]>
Date: Fri, 4 Nov 2016 12:15:55 -0700
Subject: KVM: Disable irq while unregistering user notifier
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From: Ignacio Alvarado <[email protected]>

commit 1650b4ebc99da4c137bfbfc531be4a2405f951dd upstream.

Function user_notifier_unregister should be called only once for each registered user notifier.

Function kvm_arch_hardware_disable can be executed from an IPI context which could cause a race condition with a VCPU returning to user mode and attempting to unregister the notifier.

---
 arch/x86/kvm/x86.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

@@ -199,7 +199,18 @@ static void kvm_on_user_return(struct us
 	struct kvm_shared_msrs *locals
 		= container_of(urn, struct kvm_shared_msrs, urn);
 	struct kvm_shared_msr_values *values;
+	unsigned long flags;
 
+	/*
+	 * Disabling irqs at this point since the following code could be
+	 * interrupted and executed through kvm_arch_hardware_disable()
+	 */
+	local_irq_save(flags);
+	if (locals->registered) {
+		locals->registered = false;
+		user_return_notifier_unregister(urn);
+	}
+	local_irq_restore(flags);
 	for (slot = 0; slot < shared_msrs_global.nr; ++slot) {
 		values = &locals->values[slot];
 		if (values->host != values->curr) {

@@ -207,8 +218,6 @@ static void kvm_on_user_return(struct us
 			values->curr = values->host;
 		}
 	}
-	locals->registered = false;
-	user_return_notifier_unregister(urn);
 }
 
 static void shared_msr_update(unsigned slot, u32 msr)

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/kvm-disable-irq-while-unregistering-user-notifier.patch