When using the microsoft comfort curved usb keyboard 2000, an x server crash can be triggered on exit via ctrl-alt-backspace, kill, end of session, etc.

Original reporter's bug report:  https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/205979

Xorg.0.log:  http://launchpadlibrarian.net/12838947/Xorg.0.log.old
xorg.conf:   http://launchpadlibrarian.net/12838951/xorg.conf

Program received signal SIGSEGV, Segmentation fault.
0xb7d8f39c in free () from /lib/tls/i686/cmov/libc.so.6
(gdb) backtrace full
#0 0xb7d8f39c in free () from /lib/tls/i686/cmov/libc.so.6
No symbol table info available.
#1 0x081b88f1 in Xfree (ptr=0x1180002) at ../../os/utils.c:1466
No locals.
#2 0x081aa7c0 in _XkbFreeGeomLeafElems (freeAll=<value optimized out>,
    first=<value optimized out>, count=17, num_inout=0x82652bc, sz_inout=0x82652be,
    elems=0x82652c4, elem_sz=8) at ../../xkb/XKBGAlloc.c:62
No locals.
#3 0x081aa85c in _XkbClearRow (row_in=0x0) at ../../xkb/XKBGAlloc.c:342
No locals.
#4 0x081aa29d in _XkbFreeGeomNonLeafElems (freeAll=1, first=0, count=4, num_inout=0x8264fc8,
    sz_inout=0x8264fce, elems=0x8264fd4, elem_sz=24, freeFunc=0x81aa820 <_XkbClearRow>)
    at ../../xkb/XKBGAlloc.c:119
        i = 2
        ptr = 0x82652b8 "])"
#5 0x081aa409 in SrvXkbFreeGeomRows (section=0x8264fb8, first=0, count=4, freeAll=1)
    at ../../xkb/XKBGAlloc.c:349
No locals.
#6 0x081aa451 in _XkbClearSection (section_in=0x8264fb8 "�") at ../../xkb/XKBGAlloc.c:363
No locals.
#7 0x081aa29d in _XkbFreeGeomNonLeafElems (freeAll=1, first=0, count=7, num_inout=0x82643e6,
    sz_inout=0x82643da, elems=0x82643f8, elem_sz=48, freeFunc=0x81aa410 <_XkbClearSection>)
    at ../../xkb/XKBGAlloc.c:119
        i = 3
        ptr = 0x8264fb8 "�"
#8 0x081aa3b9 in SrvXkbFreeGeomSections (geom=0x82643c0, first=0, count=7, freeAll=1)
    at ../../xkb/XKBGAlloc.c:374
No locals.
#9 0x081aaacf in SrvXkbFreeGeometry (geom=0x82643c0, which=63, freeMap=1)
    at ../../xkb/XKBGAlloc.c:443
No locals.
#10 0x081a8ca1 in SrvXkbFreeKeyboard (xkb=0x8279528, which=127, freeAll=1)
    at ../../xkb/XKBAlloc.c:328
No locals.
#11 0x08196c1f in XkbFreeInfo (xkbi=0x827a550) at ../../xkb/xkbInit.c:820
No locals.
#12 0x08085a1f in CloseDevice (dev=0x8260258) at ../../dix/devices.c:530
        k = <value optimized out>
        knext = <value optimized out>
        p = <value optimized out>
        pnext = <value optimized out>
        i = <value optimized out>
        inext = <value optimized out>
        s = <value optimized out>
        snext = <value optimized out>
        b = <value optimized out>
        bnext = <value optimized out>
        l = <value optimized out>
        lnext = <value optimized out>
#13 0x08085e79 in CloseDownDevices () at ../../dix/devices.c:626
        dev = (DeviceIntPtr) 0x1180002
        next = (DeviceIntPtr) 0x8261408
#14 0x0807474e in main (argc=8, argv=0xbf9c9f74, envp=Cannot access memory at address 0x19
) at ../../dix/main.c:472
        pScreen = <value optimized out>
        i = <value optimized out>
        error = 136163204
        xauthfile = <value optimized out>
        alwaysCheckForInput = {0, 1}

Looking at the backtrace, it appears a NULL pointer is getting passed in:

_XkbFreeGeomNonLeafElems(..., freeFunc[=_XkbClearRow]) {
   ...
    if (freeFunc) {
        ptr= *elems;
        ptr+= first*elem_sz;
        for (i=0;i<count;i++) {
            (*freeFunc)(ptr);
            ptr+= elem_sz;
        }
    }
   ...

I'm not sure how ptr comes to be NULL, but there's no check for it, and it is getting passed to freeFunc() (aka _XkbClearRow()):

#3 0x081aa85c in _XkbClearRow (row_in=0x0) at ../../xkb/XKBGAlloc.c:342

Then _XkbClearRow tries dereferencing the NULL pointer:

static void
_XkbClearRow(char *row_in)
{
XkbRowPtr row= (XkbRowPtr)row_in;

    if (row->keys!=NULL)
        XkbFreeGeomKeys(row,0,row->num_keys,True);
    return;
}

So my guess is this needs a NULL pointer check either in the calling code, or in _XkbClearRow, like

    if (row && row->keys!=NULL)