Re: [PATCH] Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()"

[Manfred Spraul <manfred@colorfullife.com>]

Hi Ioanna,

On 12/11/19 8:13 PM, Ioanna Alifieraki wrote:
> This reverts commit a97955844807e327df11aa33869009d14d6b7de0.
>
> Commit a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage
> in exit_sem()") removes a lock that is needed.

Yes, you are right, the lock is needed.

The documentation is already correct:

sem_undo_list.list_proc: undo_list->lock for write.

[...]
> Removing elements from list_id is safe for both exit_sem() and freeary()
> due to sem_lock().  Removing elements from list_proc is not safe;

Correct, removing elements is not safe.

Removing one element would be ok, as we hold sem_lock.

But if there are two elements, then we don't hold sem_lock for the 2nd 
element, and thus the list is corrupted.

> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1694779
>
> Fixes: a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()")
> Signed-off-by: Ioanna Alifieraki <ioanna-maria.alifieraki@canonical.com>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
> ---
>   ipc/sem.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/ipc/sem.c b/ipc/sem.c
> index ec97a7072413..fe12ea8dd2b3 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -2368,11 +2368,9 @@ void exit_sem(struct task_struct *tsk)
>   		ipc_assert_locked_object(&sma->sem_perm);
>   		list_del(&un->list_id);
>   
> -		/* we are the last process using this ulp, acquiring ulp->lock
> -		 * isn't required. Besides that, we are also protected against
> -		 * IPC_RMID as we hold sma->sem_perm lock now
> -		 */
> +		spin_lock(&ulp->lock);
>   		list_del_rcu(&un->list_proc);
> +		spin_unlock(&ulp->lock);
>   
>   		/* perform adjustments registered in un */
>   		for (i = 0; i < sma->sem_nsems; i++) {