Re: [PATCH] vmscan: fix potential arbitrary pointer passed to kfree in unregister_shrinker

[Michal Hocko]

[Cc Yang Shi]
On Wed 27-07-22 17:07:00, tujinjiang@xxxxxxxxxxxxx wrote:
> From: Jinjiang Tu <tujinjiang@xxxxxxxxxxxxx>
> 
> when shrinker is registered with SHRINKER_MEMCG_AWARE flag,
> register_shrinker will not initialize shrinker->nr_deferred,
> but the pointer will be passed to kfree in unregister_shrinker
> when the shrinker is unregistered. This leads to kernel crash
> when the shrinker object is dynamically allocated.

Is this a real life problem? I thought shrinkers were pre-zeroed
already. Not that we should be relying on that but it would be good to
mention whether this is a code fortification or something that we should
be really worried about.

> To fix it, this patch initialize shrinker->nr_deferred at the
> beginning of prealloc_shrinker.

It would be great to add
Fixes: 476b30a0949a ("mm: vmscan: don't need allocate shrinker->nr_deferred for memcg aware shrinkers")
 
> Signed-off-by: Jinjiang Tu <tujinjiang@xxxxxxxxxxxxx>
> ---
>  mm/vmscan.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f7d9a683e3a7..06ab5a398971 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -613,6 +613,7 @@ int prealloc_shrinker(struct shrinker *shrinker)
>  	unsigned int size;
>  	int err;
>  
> +	shrinker->nr_deferred = NULL;
>  	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
>  		err = prealloc_memcg_shrinker(shrinker);
>  		if (err != -ENOSYS)

You should be able to move it under SHRINKER_MEMCG_AWARE branch, no?

-- 
Michal Hocko
SUSE Labs