In the Linux kernel, the following vulnerability has been resolved:
debugobjects: Plug race against a concurrent OOM disable
syzbot reported a puzzling splat:
WARNING: kernel/time/hrtimer.c:443 at stub_timer+0xa/0x20
stub_timer() is installed as timer callback function in
hrtimer_fixup_assert_init(), which is invoked when
debug_object_assert_init() can't find a shadow object. In that case debug
objects emits a warning about it before invoking the fixup.
Though the provided console log lacks this warning and instead has the
following a few seconds before the splat:
<pre>
ODEBUG: Out of memory. ODEBUG disabled
</pre>
So the object was looked up in debug_object_assert_init() and the lookup
failed due a concurrent out of memory situation which disabled debug
objects and freed the shadow objects:
debug_object_assert_init()
if (!debug_objects_enabled)
return; obj = alloc();
if (!obj) {
// Out of memory
debug_objects_enabled = false;
free_objects();
obj = lookup_or_alloc();
<pre>
// The lookup failed because the other side
// removed the objects, so this returns
// an error code as the object in question
// is not statically initialized
if (!IS_ERR_OR_NULL(obj))
return;
if (!obj) {
debug_oom();
return;
}
print(...)
if (!debug_objects_enabled)
return;
fixup(...)
</pre>
The debug object splat is skipped because debug_objects_enabled is false,
but the fixup callback is invoked unconditionally, which makes the timer
disfunctional.
This is only a problem in debug_object_assert_init() and
debug_object_activate() as both have to handle statically initialized
objects and therefore must handle the error pointer return case
gracefully. All other places only handle the found/not found case and the
NULL pointer return is a signal for OOM. Otherwise they get a valid shadow
object.
Plug the hole by checking whether debug objects are still enabled before
invoking the print and fixup function in those two places.