Re: [PATCH] mm: shrinker: fix shrinker_info teardown race with expansion

[Muchun Song]

> On Jun 17, 2026, at 16:56, Qi Zheng <qi.zheng@xxxxxxxxx> wrote:
> 
> From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> 
> The expand_shrinker_info() iterates all visible memcgs under
> shrinker_mutex, including memcgs that have not finished ->css_online()
> yet.
> 
> Once pn->shrinker_info has been published, teardown must stay serialized
> with expand_shrinker_info() until that memcg is either fully online or
> no longer visible to iteration. Today alloc_shrinker_info() breaks that
> rule by dropping shrinker_mutex before freeing a partially initialized
> shrinker_info array, which may cause the following race:
> 
> CPU0                   CPU1
> ====                   ====
> 
> css_create
> --> list_add_tail_rcu(&css->sibling, &parent_css->children);
>    online_css
>    --> mem_cgroup_css_online
>        --> alloc_shrinker_info
>            --> alloc node0 info
>                rcu_assign_pointer(C->node0->shrinker_info, old0)
>                alloc node1 info -> FAIL -> goto err
>                mutex_unlock(shrinker_mutex)
> 
>                       shrinker_alloc()
>                       --> shrinker_memcg_alloc
>                           --> mutex_lock(shrinker_mutex)
>                               expand_shrinker_info
>                               --> mem_cgroup_iter see the memcg
>                                   expand_one_shrinker_info
>                                   --> old0 = C->node0->shrinker_info
>                                       memcpy(new->unit, old0->unit, ...);
> 
>                free_shrinker_info
>                --> kvfree(old0);
> 
>                                       /* double free !! */
>                                       kvfree_rcu(old0, rcu);
> 
> The same problem exists later in mem_cgroup_css_online(). If
> alloc_shrinker_info() succeeds but a subsequent objcg allocation fails,
> the free_objcg -> free_shrinker_info() unwind path tears down the already
> published pn->shrinker_info arrays without shrinker_mutex. The
> expand_one_shrinker_info() can race with that teardown in the same way,
> leading to use-after-free or double-free of the old shrinker_info.
> 
> Fix this by serializing shrinker_info teardown with shrinker_mutex, and by
> keeping alloc_shrinker_info() error cleanup inside the locked section.
> 
> Fixes: 307bececcd12 ("mm: shrinker: add a secondary array for shrinker_info::{map, nr_deferred}")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>

Acked-by: Muchun Song <muchun.song@xxxxxxxxx>

Thanks.