[PATCH] debugobjects: Plug race against a concurrent OOM disable

[Thomas Gleixner]

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.

Fixes: b84d435cc228 ("debugobjects: Extend to assert that an object is initialized")
Reported-by: syzbot+5e8dda76ca21dae314b6@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx

---
 lib/debugobjects.c |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -894,6 +894,14 @@ int debug_object_activate(void *addr, co
 	}
 
 	raw_spin_unlock_irqrestore(&db->lock, flags);
+
+	/*
+	 * lookup_object_or_alloc() might have raced with a concurrent
+	 * allocation failure which disabled debug objects.
+	 */
+	if (!debug_objects_enabled)
+		return 0;
+
 	debug_print_object(&o, "activate");
 
 	switch (o.state) {
@@ -1071,6 +1079,15 @@ void debug_object_assert_init(void *addr
 		return;
 	}
 
+	/*
+	 * lookup_object_or_alloc() might have raced with a concurrent
+	 * allocation failure which disabled debug objects. Don't run the fixup
+	 * as it might turn a valid object useless. See for example
+	 * hrtimer_fixup_assert_init().
+	 */
+	if (!debug_objects_enabled)
+		return;
+
 	/* Object is neither tracked nor static. It's not initialized. */
 	debug_print_object(&o, "assert_init");
 	debug_object_fixup(descr->fixup_assert_init, addr, ODEBUG_STATE_NOTAVAILABLE);