Created attachment 50840 [details]
Simple reproducer program

Description

FreeSelectionProperty() does not check for the count of items in array and
relies on a NULL terminated list, which can cause libXt to crash if
FreeSelectionProperty() follows a call to GetSelectionProperty() which
reallocates the array.

Version-Release number of selected component (if applicable):

All including current git

How reproducible:

Randomly, requires a selection in gtk+ version < 2.16.6 

Steps to Reproduce:

  1. Use a system with gtk+ version < 2.16.6 (see additional info as to why)
  2. Download and save the attached reproducer in C "selectiontest.c"
  3. Build the reproducer with:

   $ gcc -o selectiontest -Wall -Wextra -lXm -lXt -lX11 selectiontest.c

  4. Run the reproducer

   $ ./selectiontest

  5. Select some text with the mouse in another gtk application such as gedit
     or gnome-terminal

  6. Click on the button "Click here" in the reproducer while the selection
     is still active in the gtk+ application

Actual results:

  The reproducer crashes in Xt code FreeSelectionProperty()

Expected results:

  No crash

Additional info:

Valgrind shows the following error:

  ==18507== Invalid read of size 8
  ==18507==    at 0x532F6BE: FreeSelectionProperty (Selection.c:251)
  ==18507==    by 0x532FD90: HandleSelectionReplies (Selection.c:1400)
  ==18507==    by 0x531C1C8: XtDispatchEventToWidget (Event.c:901)
  ==18507==    by 0x531C563: _XtDefaultDispatcher (Event.c:1344)
  ==18507==    by 0x531C853: XtDispatchEvent (Event.c:1423)
  ==18507==    by 0x531CCFA: XtAppMainLoop (Event.c:1560)
  ==18507==    by 0x400A40: main (in /home/ofourdan/selectiontest)
  ==18507==  Address 0x85d7e70 is 0 bytes after a block of size 64 alloc'd
  ==18507==    at 0x4C26582: realloc (vg_replace_malloc.c:525)
  ==18507==    by 0x530D532: XtRealloc (Alloc.c:189)
  ==18507==    by 0x532E550: GetSelectionProperty (Selection.c:227)
  ==18507==    by 0x5330300: GetSelectionValues.part.4 (Selection.c:1686)
  ==18507==    by 0x5331FF1: XtGetSelectionValues (Selection.c:1636)
  ==18507==    by 0x400989: cb_clicked (in /home/ofourdan/selectiontest)
  ==18507==    by 0x530E0AF: XtCallCallbackList (Callback.c:625)
  ==18507==    by 0x4F13C76: ActivateCommon (PushB.c:2247)
  ==18507==    by 0x5342EAC: HandleActions (TMstate.c:645)
  ==18507==    by 0x5343CB6: _XtTranslateEvent (TMstate.c:1071)
  ==18507==    by 0x531C052: XtDispatchEventToWidget (Event.c:906)
  ==18507==    by 0x531C795: _XtDefaultDispatcher (Event.c:1367)

The original problem relies in gtk+ which creates an atom for "NONE", this is GNOME bug 580511:

    https://bugzilla.gnome.org/show_bug.cgi?id=580511

Which was fixed with this git commit 6aa63385:
   
    http://git.gnome.org/browse/gtk+/commit/gdk/x11/gdkproperty-x11.c?id=6aa63385789fd28e61eb4de05312e2f1316cf6d0

However, even if Xt is not the source of the problem, it should not crash because of another (broken) application.

Proposed patch attached, which checks for the "propCount" value in FreeSelectionProperty()