[Xen-changelog] [xen-4.0-testing] VCPU/timers: Prevent overflow in calculations, leading to DoS vulnerability

[Xen patchbot-4.0-testing <patchbot@xxxxxxx>]

# HG changeset patch
# User Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
# Date 1352893075 0
# Node ID 6b9809dc1e860b73d644d7b89435df14f0723657
# Parent  1d1538beeadadebc0665bae56cfce6d58c51dc09
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@xxxxxxxxxx>
Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Committed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

xen-unstable changeset: 26148:bf58b94b3cef
Backport-requested-by: security@xxxxxxx
Committed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---


diff -r 1d1538beeada -r 6b9809dc1e86 xen/common/domain.c
--- a/xen/common/domain.c       Wed Sep 12 11:18:03 2012 +0100
+++ b/xen/common/domain.c       Wed Nov 14 11:37:55 2012 +0000
@@ -823,6 +823,9 @@ 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);
 
diff -r 1d1538beeada -r 6b9809dc1e86 xen/include/xen/time.h
--- a/xen/include/xen/time.h    Wed Sep 12 11:18:03 2012 +0100
+++ b/xen/include/xen/time.h    Wed Nov 14 11:37:55 2012 +0000
@@ -52,6 +52,8 @@ 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);

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