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 __do_softirq().  The
affected PCPU will stay in an infinite loop until Xen is killed by the
watchdog (if enabled).

This is a security problem, XSA-20 / CVE-2012-4535.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

xen/common/domain.c

@@ -903,6 +903,9 @@ long do_vcpu_op(int cmd, int vcpuid, XEN_GUEST_HANDLE_PARAM(void) arg)
         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);
 

xen/include/xen/time.h

@@ -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);