KVM: x86: hyper-v: Don't auto-enable stimer on write from user-space

Don't apply the stimer's counter side effects when modifying its
value from user-space, as this may trigger spurious interrupts.

For example:
 - The stimer is configured in auto-enable mode.
 - The stimer's count is set and the timer enabled.
 - The stimer expires, an interrupt is injected.
 - The VM is live migrated.
 - The stimer config and count are deserialized, auto-enable is ON, the
   stimer is re-enabled.
 - The stimer expires right away, and injects an unwarranted interrupt.

Cc: stable@vger.kernel.org
Fixes: 1f4b34f825e8 ("kvm/x86: Hyper-V SynIC timers")
Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20231017155101.40677-1-nsaenz@amazon.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 7c2dac6824e262e0e82f1ed1d7bd564598006713..238afd7335e46d7fcce8d8ad096136cf35bf70b1 100644 (file)
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -727,10 +727,12 @@ static int stimer_set_count(struct kvm_vcpu_hv_stimer *stimer, u64 count,
 
        stimer_cleanup(stimer);
        stimer->count = count;
-       if (stimer->count == 0)
-               stimer->config.enable = 0;
-       else if (stimer->config.auto_enable)
-               stimer->config.enable = 1;
+       if (!host) {
+               if (stimer->count == 0)
+                       stimer->config.enable = 0;
+               else if (stimer->config.auto_enable)
+                       stimer->config.enable = 1;
+       }
 
        if (stimer->config.enable)
                stimer_mark_pending(stimer, false);