In the Linux kernel, the following vulnerability has been resolved: debugobjects: Plug race...

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:

 ODEBUG: Out of memory. ODEBUG disabled

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();

    // 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(...)

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.