CVE-2012-4535
Timer overflow DoS vulnerability
A guest which sets a VCPU with an inappropriate deadline can cause an infinite loop in Xen, blocking the affected physical CPU indefinitely.
lack of check (timer overflow)
http://xenbits.xen.org/hg/xen-4.2-testing.hg/rev/788af5959f69
VCPU/timers: Prevent overflow in calculations, leading to DoS vulnerability
The timer action for a vcpu periodic timer is to calculate the next expiry time, and to reinsert itself into the timer queue. If the deadline ends up in the past, Xen never leaves _dosoftirq(). The affected PCPU will stay in an infinite loop until Xen is killed by the watchdog (if enabled).
--- a/xen/common/domain.c Wed Nov 14 10:40:41 2012 +0100
+++ b/xen/common/domain.c Wed Nov 14 11:33:15 2012 +0000
@@ -882,6 +882,9 @@ long do_vcpu_op(int cmd, int vcpuid, XEN
if ( set.period_ns < MILLISECS(1) )
return -EINVAL;
+ if ( set.period_ns > STIME_DELTA_MAX )
+ return -EINVAL;
+
v->periodic_period = set.period_ns;
vcpu_force_reschedule(v);
--- a/xen/include/xen/time.h Wed Nov 14 10:40:41 2012 +0100
+++ b/xen/include/xen/time.h Wed Nov 14 11:33:15 2012 +0000
@@ -55,6 +55,8 @@ struct tm gmtime(unsigned long t);
#define MILLISECS(_ms) ((s_time_t)((_ms) * 1000000ULL))
#define MICROSECS(_us) ((s_time_t)((_us) * 1000ULL))
#define STIME_MAX ((s_time_t)((uint64_t)~0ull>>1))
+/* Chosen so (NOW() + delta) wont overflow without an uptime of 200 years */
+#define STIME_DELTA_MAX ((s_time_t)((uint64_t)~0ull>>2))
extern void update_vcpu_system_time(struct vcpu *v);
extern void update_domain_wallclock_time(struct domain *d);A malicious guest administrator can trigger the bug. If the Xen watchdog is enabled, the whole system will crash. Otherwise the guest can cause the system to become completely unresponsive.
DoS