Re: [PATCH] mm/vmalloc: fix KMSAN uninit in decay_va_pool_node list handling

[Uladzislau Rezki]

On Fri, Apr 03, 2026 at 03:52:03PM +0800, chenyichong wrote:
> Prevent decay_va_pool_node from overwriting concurrent repopulation of
> vmap_node pool[i].head while purging. Read/reset pool[i].len under
> pool_lock and splice leftover vmap_area nodes back into the pool
> instead of replacing the list.
> 
> Reported-by: syzbot+37b7f6cd519f7fb8d32a@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=37b7f6cd519f7fb8d32a
> Fixes: 7679ba6b36db ("mm: vmalloc: add a shrinker to drain vmap pools")
> Signed-off-by: chenyichong <chenyichong@xxxxxxxxxxxxx>
> ---
>  mm/vmalloc.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index ecbac900c35f..72fb60553a71 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2233,10 +2233,9 @@ decay_va_pool_node(struct vmap_node *vn, bool full_decay)
>  		/* Detach the pool, so no-one can access it. */
>  		spin_lock(&vn->pool_lock);
>  		list_replace_init(&vn->pool[i].head, &tmp_list);
> -		spin_unlock(&vn->pool_lock);
> -
>  		pool_len = n_decay = vn->pool[i].len;
>  		WRITE_ONCE(vn->pool[i].len, 0);
> +		spin_unlock(&vn->pool_lock);
>  
>  		/* Decay a pool by ~25% out of left objects. */
>  		if (!full_decay)
> @@ -2259,8 +2258,14 @@ decay_va_pool_node(struct vmap_node *vn, bool full_decay)
>  		 */
>  		if (!list_empty(&tmp_list)) {
>  			spin_lock(&vn->pool_lock);
> -			list_replace_init(&tmp_list, &vn->pool[i].head);
> -			WRITE_ONCE(vn->pool[i].len, pool_len);
> +			/*
> +			 * Merge leftover areas back into the pool rather than
> +			 * replacing the whole list. A concurrent allocator can
> +			 * repopulate vn->pool[i].head while we are decaying
> +			 * tmp_list, and replacing would drop those nodes.
> +			 */
> +			list_splice_tail_init(&tmp_list, &vn->pool[i].head);
> +			WRITE_ONCE(vn->pool[i].len, vn->pool[i].len + pool_len);
>
"A concurrent allocator can repopulate..." - Where is it done? Probably
you meant something different.

--
Uladzislau Rezki