Re: [PATCH 1/1] ipc/shm: serialize orphan cleanup with shm_nattch updates

[Andrew Morton]

On Thu, 30 Apr 2026 13:21:34 +0800 Ren Wei <n05ec@xxxxxxxxxx> wrote:

> From: Yilin Zhu <zylzyl2333@xxxxxxxxx>
> 
> shm_destroy_orphaned() walks the shm idr under shm_ids(ns).rwsem, but
> that does not serialize all fields tested by shm_may_destroy().  In
> particular, shm_nattch is updated while holding shm_perm.lock, and attach
> paths can do that without holding the rwsem.
> 
> Do not decide that an orphaned segment is unused before taking the object
> lock.  Move the shm_may_destroy() check under shm_perm.lock, matching the
> other destroy paths, and unlock the segment when it no longer qualifies
> for removal.

Thanks.

> Fixes: 4c677e2eefdb ("shm: optimize locking and ipc_namespace getting")

Let's cc more people who were involved in 4c677e2eefdb.

And Davidlohr, who might have opinions.

> Cc: stable@xxxxxxxxxx
> Reported-by: Yuan Tan <yuantan098@xxxxxxxxx>
> Reported-by: Yifan Wu <yifanwucs@xxxxxxxxx>
> Reported-by: Juefei Pu <tomapufckgml@xxxxxxxxx>
> Reported-by: Xin Liu <bird@xxxxxxxxxx>
> Signed-off-by: Yilin Zhu <zylzyl2333@xxxxxxxxx>
> Signed-off-by: Ren Wei <n05ec@xxxxxxxxxx>
> ---
>  ipc/shm.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/ipc/shm.c b/ipc/shm.c
> index a95dae447707..b3e8a58e177d 100644
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -418,15 +418,17 @@ static int shm_try_destroy_orphaned(int id, void *p, void *data)
>  	 * We want to destroy segments without users and with already
>  	 * exit'ed originating process.
>  	 *
> -	 * As shp->* are changed under rwsem, it's safe to skip shp locking.
> +	 * shm_nattch can be changed under shm_perm.lock without holding the
> +	 * rwsem, so take the object lock before checking shm_may_destroy().
>  	 */
>  	if (!list_empty(&shp->shm_clist))
>  		return 0;
>  
> -	if (shm_may_destroy(shp)) {
> -		shm_lock_by_ptr(shp);
> +	shm_lock_by_ptr(shp);
> +	if (shm_may_destroy(shp))
>  		shm_destroy(ns, shp);
> -	}
> +	else
> +		shm_unlock(shp);
>  	return 0;
>  }