[Xen-devel] [PATCH] x86/monitor: minor left-shift undefined behavior checks

[Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>]

This minor patch adds a range-check to avoid left-shift caused undefined
behavior. Also replaces '1 <<' w/ '1U <<' @ x86 monitor.h in an effort to avoid
a future potential '1 << 31' that would cause a similar issue.

Signed-off-by: Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>
---
 xen/arch/x86/monitor.c        | 13 +++++++++----
 xen/include/asm-x86/monitor.h | 10 +++++-----
 2 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/monitor.c b/xen/arch/x86/monitor.c
index a507edb..b4bd008 100644
--- a/xen/arch/x86/monitor.c
+++ b/xen/arch/x86/monitor.c
@@ -32,10 +32,15 @@ int arch_monitor_domctl_event(struct domain *d,
     {
     case XEN_DOMCTL_MONITOR_EVENT_WRITE_CTRLREG:
     {
-        unsigned int ctrlreg_bitmask =
-            monitor_ctrlreg_bitmask(mop->u.mov_to_cr.index);
-        bool_t old_status =
-            !!(ad->monitor.write_ctrlreg_enabled & ctrlreg_bitmask);
+        unsigned int ctrlreg_bitmask;
+        bool_t old_status;
+
+        /* sanity check: avoid left-shift undefined behavior */
+        if ( unlikely(mop->u.mov_to_cr.index > 31) )
+            return -EINVAL;
+
+        ctrlreg_bitmask = monitor_ctrlreg_bitmask(mop->u.mov_to_cr.index);
+        old_status = !!(ad->monitor.write_ctrlreg_enabled & ctrlreg_bitmask);
 
         if ( unlikely(old_status == requested_status) )
             return -EEXIST;
diff --git a/xen/include/asm-x86/monitor.h b/xen/include/asm-x86/monitor.h
index c789f71..f1bf4bd 100644
--- a/xen/include/asm-x86/monitor.h
+++ b/xen/include/asm-x86/monitor.h
@@ -40,14 +40,14 @@ static inline uint32_t arch_monitor_get_capabilities(struct 
domain *d)
     if ( !is_hvm_domain(d) || !cpu_has_vmx )
         return capabilities;
 
-    capabilities = (1 << XEN_DOMCTL_MONITOR_EVENT_WRITE_CTRLREG) |
-                   (1 << XEN_DOMCTL_MONITOR_EVENT_MOV_TO_MSR) |
-                   (1 << XEN_DOMCTL_MONITOR_EVENT_SOFTWARE_BREAKPOINT) |
-                   (1 << XEN_DOMCTL_MONITOR_EVENT_GUEST_REQUEST);
+    capabilities = (1U << XEN_DOMCTL_MONITOR_EVENT_WRITE_CTRLREG) |
+                   (1U << XEN_DOMCTL_MONITOR_EVENT_MOV_TO_MSR) |
+                   (1U << XEN_DOMCTL_MONITOR_EVENT_SOFTWARE_BREAKPOINT) |
+                   (1U << XEN_DOMCTL_MONITOR_EVENT_GUEST_REQUEST);
 
     /* Since we know this is on VMX, we can just call the hvm func */
     if ( hvm_is_singlestep_supported() )
-        capabilities |= (1 << XEN_DOMCTL_MONITOR_EVENT_SINGLESTEP);
+        capabilities |= (1U << XEN_DOMCTL_MONITOR_EVENT_SINGLESTEP);
 
     return capabilities;
 }
-- 
2.5.0


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