[3.11.y.z extended stable] Patch "KVM: x86: Fix potential divide by 0 in lapic (CVE-2013-6367)" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

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

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From f0899607dcdd8d445a9f23f0018711276cd7ddbb Mon Sep 17 00:00:00 2001
From: Andy Honig <ahonig at google.com>
Date: Tue, 19 Nov 2013 14:12:18 -0800
Subject: KVM: x86: Fix potential divide by 0 in lapic (CVE-2013-6367)

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 at google.com>
Signed-off-by: Andrew Honig <ahonig at google.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 arch/x86/kvm/lapic.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index afc1124..adbcc08 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -855,7 +855,8 @@ 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);
--
1.8.3.2