linux-kernel - Re: [PATCH] mm: kmem: properly initialize local objcg variable in current_obj

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Date:   Thu, 16 Nov 2023 08:04:18 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Erhard Furtner <erhard_f@...lbox.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Dennis Zhou <dennis@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Muchun Song <muchun.song@...ux.dev>, stable@...r.kernel.org
Subject: Re: [PATCH] mm: kmem: properly initialize local objcg variable in
 current_obj_cgroup()

On 11/16/23 03:51, Roman Gushchin wrote:
> Actually the problem is caused by uninitialized local variable in
> current_obj_cgroup(). If the root memory cgroup is set as an active
> memory cgroup for a charging scope (as in the trace, where systemd
> tries to create the first non-root cgroup, so the parent cgroup is
> the root cgroup), the "for" loop is skipped and uninitialized objcg is
> returned, causing a panic down the accounting stack.
> 
> The fix is trivial: initialize the objcg variable to NULL
> unconditionally before the "for" loop.
> 
> Fixes: e86828e5446d ("mm: kmem: scoped objcg protection")
> Reported-by: Erhard Furtner <erhard_f@...lbox.org>
> Closes: https://github.com/ClangBuiltLinux/linux/issues/1959
> Signed-off-by: Roman Gushchin (Cruise) <roman.gushchin@...ux.dev>
> Cc: Shakeel Butt <shakeelb@...gle.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Dennis Zhou <dennis@...nel.org>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Muchun Song <muchun.song@...ux.dev>
> Cc: stable@...r.kernel.org

Acked-by: Vlastimil Babka <vbabka@...e.cz>

We could also do this to make it less confusing?

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 774bd6e21e27..a08bcec661b6 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3175,7 +3175,6 @@ __always_inline struct obj_cgroup *current_obj_cgroup(void)
 		objcg = rcu_dereference_check(memcg->objcg, 1);
 		if (likely(objcg))
 			break;
-		objcg = NULL;
 	}
 
 	return objcg;


> ---
>  mm/memcontrol.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 774bd6e21e27..b138501e6489 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3165,6 +3165,7 @@ __always_inline struct obj_cgroup *current_obj_cgroup(void)
>  	return NULL;
>  
>  from_memcg:
> +	objcg = NULL;
>  	for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg)) {
>  		/*
>  		 * Memcg pointer is protected by scope (see set_active_memcg())