[PATCH 3.12 017/118] KVM: x86: Fix potential divide by 0 in lapic (CVE-2013-6367)

Greg Kroah-Hartman
2013-12-18T21:48:33Z

From: Andy Honig <ahonig@google.com>

commit b963a22e6d1a266a67e9eecc88134713fd54775c upstream.

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>
Signed-off-by: Andrew Honig <ahonig@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/kvm/lapic.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -841,7 +841,8 @@ static u32 apic_get_tmcct(struct kvm_lap
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);