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

[Brad Figg]

On 01/09/2014 09:10 AM, Luis Henriques wrote:
> From: Andy Honig <ahonig at google.com>
> 
> CVE-2013-6367
> 
> BugLink: http://bugs.launchpad.net/bugs/1261566
> 
> 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>
> Cc: stable at vger.kernel.org
> Signed-off-by: Andrew Honig <ahonig at google.com>
> Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
> (back ported from commit b963a22e6d1a266a67e9eecc88134713fd54775c)
> 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 8dfeaaa..b77857f 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -519,7 +519,8 @@ static u32 apic_get_tmcct(struct kvm_lapic *apic)
>  	ASSERT(apic != NULL);
>  
>  	/* if initial count is 0, current count should also be 0 */
> -	if (apic_get_reg(apic, APIC_TMICT) == 0)
> +	if (apic_get_reg(apic, APIC_TMICT) == 0 ||
> +		apic->lapic_timer.period == 0)
>  		return 0;
>  
>  	remaining = hrtimer_get_remaining(&apic->lapic_timer.timer);
> 


-- 
Brad Figg brad.figg at canonical.com http://www.canonical.com