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.

https://git.kernel.org/stable/c/1f4f02b336c3be125c8fcf87df73db2e0e028b8b

https://git.kernel.org/stable/c/203a965bf2ab43130778d8214fb0c3c8c2d19cdf

https://git.kernel.org/stable/c/23da32e88627e63e0864f59f4c63a2dc0ab851a3

https://git.kernel.org/stable/c/2d5e320b7ab9b25229ac4331541964a58b5e1d29

https://git.kernel.org/stable/c/b81dde13cc163450dcb402dcc915ef13ba241e01

https://git.kernel.org/stable/c/c00164c9e7fa6145886ad666806cb5347895de5c

https://git.kernel.org/stable/c/d663fbf28b2eebe665bb9cf828d7d528e5a8707e

https://git.kernel.org/stable/c/e2e255d07723c330dded8e576ce28a8d23a692ce

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