Bug#744303: xserver-xorg-core: Segmentation fault when receiving a SIGIO in DeepCopyDeviceClasses

[Steven McDonald <steven@steven-mcdonald.id.au>]

Package: xserver-xorg-core
Version: 2:1.15.0.901-1
Severity: important
Tags: upstream patch

Hi!

I've been seeing sporadic (anywhere from once every few days to 3-4
times a day) crashes and freezes in X. The problematic behaviour isn't
always the same, but I chose a particular incident to debug, and found
that X was segfaulting in updateMotionHistory, on line 575 of
dix/getevents.c.

After some further investigation, I found that the bug was being
triggered when a SIGIO was received in DeepCopyPointerClasses, between
the AllocValuatorClass call (line 540) and updating the to->valuator
pointer (line 545). AllocValuatorClass calls realloc() on to->valuator,
so between these lines, it's not guaranteed to point to allocated
memory.

It seems the SIGIO handler is calling updateMotionHistory, which is
reading the memory pointed to by to->valuator and getting a wrong value
for last_motion, which updates buff to point to wildly the wrong place
and thus generates a segfault when a memcpy() is done into buff.

This diagnosis was performed on my work machine, which is the only
place I have been able to reproduce the problem, so I don't have a
backtrace or a relevant Xorg.0.log handy. Let me know if you'd like
that information, and I can supply it on Monday.

I am attaching a patch which I've been running on that machine for the
past three days, and haven't yet observed any more crashing or freezing
behaviour. The patch simply calls OsBlockSIGIO while
DeepCopyDeviceClasses is in progress, as the state of the X server's
device data structures is not guaranteed to be in a consistent state
during that time.

Please let me know if you need any further information.

Thanks,
Steven.

Block SIGIOs while copying device classes around, so that we don't try
to do anything with input when devices are in an inconsistent state.
--- a/Xi/exevents.c
+++ b/Xi/exevents.c
@@ -661,6 +661,8 @@
 DeepCopyDeviceClasses(DeviceIntPtr from, DeviceIntPtr to,
                       DeviceChangedEvent *dce)
 {
+    OsBlockSIGIO();
+
     /* generic feedback classes, not tied to pointer and/or keyboard */
     DeepCopyFeedbackClasses(from, to);
 
@@ -668,6 +670,8 @@
         DeepCopyKeyboardClasses(from, to);
     if ((dce->flags & DEVCHANGE_POINTER_EVENT))
         DeepCopyPointerClasses(from, to);
+
+    OsReleaseSIGIO();
 }
 
 /**