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KVM: x86: Fix potential divide by 0 in lapic (CVE-2013-6367)

Under guest controllable circumstances apic_get_tmcct will execute a
divide by zero and cause a crash.  If the guest cpuid support
tsc deadline timers and performs the following sequence of requests
the host will crash.
- Set the mode to periodic
- Set the TMICT to 0
- Set the mode bits to 11 (neither periodic, nor one shot, nor tsc deadline)
- Set the TMICT to non-zero.
Then the lapic_timer.period will be 0, but the TMICT will not be.  If the
guest then reads from the TMCCT then the host will perform a divide by 0.

This patch ensures that if the lapic_timer.period is 0, then the division
does not occur.

Reported-by: Andrew Honig <>
Signed-off-by: Andrew Honig <>
Signed-off-by: Paolo Bonzini <>
1 parent 338c7db commit b963a22e6d1a266a67e9eecc88134713fd54775c Andy Honig committed with bonzini Nov 19, 2013
Showing with 2 additions and 1 deletion.
  1. +2 −1 arch/x86/kvm/lapic.c
3 arch/x86/kvm/lapic.c
@@ -841,7 +841,8 @@ static u32 apic_get_tmcct(struct kvm_lapic *apic)
ASSERT(apic != NULL);
/* if initial count is 0, current count should also be 0 */
- if (kvm_apic_get_reg(apic, APIC_TMICT) == 0)
+ if (kvm_apic_get_reg(apic, APIC_TMICT) == 0 ||
+ apic->lapic_timer.period == 0)
return 0;
remaining = hrtimer_get_remaining(&apic->lapic_timer.timer);

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