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

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

[Breno Leitao]

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.

Fixes: e93d4166b40a ("mm: memcg: put cgroup v1-specific code under a config option")
Cc: stable@vger.kernel.org
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 mm/memcontrol-v1.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol-v1.h b/mm/memcontrol-v1.h
index f92f81108d5ed..4fa6e2bc8413f 100644
--- a/mm/memcontrol-v1.h
+++ b/mm/memcontrol-v1.h
@@ -107,7 +107,11 @@ static inline void memcg1_remove_from_trees(struct mem_cgroup *memcg) {}
 static inline void memcg1_soft_limit_reset(struct mem_cgroup *memcg) {}
 static inline void memcg1_css_offline(struct mem_cgroup *memcg) {}
 
-static inline bool memcg1_oom_prepare(struct mem_cgroup *memcg, bool *locked) { return true; }
+static inline bool memcg1_oom_prepare(struct mem_cgroup *memcg, bool *locked)
+{
+	*locked = false;
+	return true;
+}
 static inline void memcg1_oom_finish(struct mem_cgroup *memcg, bool locked) {}
 static inline void memcg1_oom_recover(struct mem_cgroup *memcg) {}
 

---
base-commit: 4e5dfb7c84012007c3c7061126491bbc92d71bf1
change-id: 20260626-memcg-oom-uninit-locked-5ec79dff4396

Best regards,
-- 
Breno Leitao <leitao@debian.org>

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

[Joshua Hahn]

Hi Breno, I hope you are doing well : -)
Woah, thank you for finding and fixing this bug! 

> Fixes: e93d4166b40a ("mm: memcg: put cgroup v1-specific code under a config option")
> Cc: stable@vger.kernel.org
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
>  mm/memcontrol-v1.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol-v1.h b/mm/memcontrol-v1.h
> index f92f81108d5ed..4fa6e2bc8413f 100644
> --- a/mm/memcontrol-v1.h
> +++ b/mm/memcontrol-v1.h
> @@ -107,7 +107,11 @@ static inline void memcg1_remove_from_trees(struct mem_cgroup *memcg) {}
>  static inline void memcg1_soft_limit_reset(struct mem_cgroup *memcg) {}
>  static inline void memcg1_css_offline(struct mem_cgroup *memcg) {}
>  
> -static inline bool memcg1_oom_prepare(struct mem_cgroup *memcg, bool *locked) { return true; }
> +static inline bool memcg1_oom_prepare(struct mem_cgroup *memcg, bool *locked)
> +{
> +	*locked = false;
> +	return true;
> +}
>  static inline void memcg1_oom_finish(struct mem_cgroup *memcg, bool locked) {}
>  static inline void memcg1_oom_recover(struct mem_cgroup *memcg) {}

Part of me wonders if we should just initialize locked = false in the
caller (mem_cgroup_oom) as to not make the stub have side effects,
but your chnage looks correct and this is a fix so perhaps that is
not so important.

Looks good to me! Thank you again Breno : -)

Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>

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

[Breno Leitao]

Hello Joshua,

On Fri, Jun 26, 2026 at 06:56:11AM -0700, Joshua Hahn wrote:
> Part of me wonders if we should just initialize locked = false in the
> caller (mem_cgroup_oom) as to not make the stub have side effects,
> but your chnage looks correct and this is a fix so perhaps that is
> not so important.

Nice, your approach seems to be even better than my silly one. I will
wait for further review and respin with your approach.

Thanks,
--breno

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

[Johannes Weiner]

On Fri, Jun 26, 2026 at 05:43:02AM -0700, Breno Leitao 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.
> 
> Fixes: e93d4166b40a ("mm: memcg: put cgroup v1-specific code under a config option")
> Cc: stable@vger.kernel.org
> Signed-off-by: Breno Leitao <leitao@debian.org>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

I prefer this way over the idea to initialize in the caller. For the
actual implementation, the protocol is that the thing is initialized
when the function returns true. This version of the fix maintains that
for the dummy as well:

> ---
>  mm/memcontrol-v1.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol-v1.h b/mm/memcontrol-v1.h
> index f92f81108d5ed..4fa6e2bc8413f 100644
> --- a/mm/memcontrol-v1.h
> +++ b/mm/memcontrol-v1.h
> @@ -107,7 +107,11 @@ static inline void memcg1_remove_from_trees(struct mem_cgroup *memcg) {}
>  static inline void memcg1_soft_limit_reset(struct mem_cgroup *memcg) {}
>  static inline void memcg1_css_offline(struct mem_cgroup *memcg) {}
>  
> -static inline bool memcg1_oom_prepare(struct mem_cgroup *memcg, bool *locked) { return true; }
> +static inline bool memcg1_oom_prepare(struct mem_cgroup *memcg, bool *locked)
> +{
> +	*locked = false;
> +	return true;
> +}
>  static inline void memcg1_oom_finish(struct mem_cgroup *memcg, bool locked) {}
>  static inline void memcg1_oom_recover(struct mem_cgroup *memcg) {}