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Subject[PATCH 3.2 092/126] KVM: Disable irq while unregistering user notifier
3.2.85-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Ignacio Alvarado <ikalvarado@google.com>

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.

Signed-off-by: Ignacio Alvarado <ikalvarado@google.com>
Fixes: 18863bdd60f8 ("KVM: x86 shared msr infrastructure")
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/x86/kvm/x86.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -172,7 +172,18 @@ 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) {
@@ -180,8 +191,6 @@ 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)
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