Re: [PATCH v2] mm: memcg: initialize *locked in memcg1_oom_prepare() stub

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Jul 16, 2026 at 05:39:49PM -0700, Andrew Morton wrote:
> On Thu, 16 Jul 2026 06:42:18 -0700 Breno Leitao <leitao@debian.org> wrote:
> 
> > mem_cgroup_oom() passes an uninitialized "locked" to memcg1_oom_prepare()
> > and reads it back in memcg1_oom_finish():
> > 
> > 	bool locked, ret;
> > 	...
> > 	if (!memcg1_oom_prepare(memcg, &locked))
> > 		return false;
> > 	ret = mem_cgroup_out_of_memory(memcg, mask, order);
> > 	memcg1_oom_finish(memcg, locked);
> > 
> > This relies on memcg1_oom_prepare() setting *locked whenever it returns
> > true.  The CONFIG_MEMCG_V1=y version does, but the stub used when
> > CONFIG_MEMCG_V1=n returns true without touching *locked, so
> > memcg1_oom_finish() consumes an uninitialized value.  On a memcg OOM this
> > is reported by UBSAN:
> > 
> >   UBSAN: invalid-load in mm/memcontrol.c:1932:27
> >   load of value 0 is not a valid value for type 'bool' (aka '_Bool')
> > 
> > Initialize *locked to false in the stub; with cgroup v1 compiled out
> > there is no OOM lock to take.
> 
> Thanks.
> 
> Sashiko, as is its wont, reminds us that we all suck:
> 	https://sashiko.dev/#/patchset/20260716-memcg-oom-uninit-locked-v2-1-63631d878eb4@debian.org
> 
> Does this potential memcg issue look legit?

Yes, it looks legit to me. The task is meant to be put to sleep after
this until current->memcg_in_oom is cleared. But sleep-and-clear
happens on userspace resume and there can be multiple allocs before.

Is it worth fixing? It's a deprecated feature inside a deprecated
feature (6df4ad704707 ("memcg: initiate deprecation of oom_control"),
August 2024). Maybe we just ought to delete all this...