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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 <ahonig@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Andrew Honig <ahonig@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Andy Honig 2013-11-19 14:12:18 -08:00 committed by Paolo Bonzini
parent 338c7dbadd
commit b963a22e6d
1 changed files with 2 additions and 1 deletions
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3
arch/x86/kvm/lapic.c
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@ -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);