Re: [PATCH v2] exit: hold a reference to thread_pid across proc_flush_pid

[Oleg Nesterov <oleg@redhat.com>]

On 08/31, Daehyeon Ko wrote:
>
> Commit 0a36bad01731 ("release_task: kill the no longer needed
> get/put_pid(thread_pid)") removed the reference around proc_flush_pid().
> It assumed that free_pids(post.pids) at the end of release_task() would
> keep thread_pid alive until then.
>
> That assumption is wrong.  __change_pid() only records a detached PID in
> post.pids when pid_has_task() is false for every PIDTYPE.  If another task
> still uses the exiting task's PID as its process group or session ID,
> __unhash_process() removes the exiting task's PIDTYPE_PID link but leaves
> the PID out of post.pids.  release_task() therefore holds no reference to
> it after dropping tasklist_lock.
>
> The other task can then remove the remaining PIDTYPE links.  Its
> free_pids() call schedules delayed_put_pid(), and the RCU callback can free
> the PID before the first release_task() reaches proc_flush_pid().

Much better, thanks. The comment above get_pid(task_pid(p)) looks better too.

Acked-by: Oleg Nesterov <oleg@redhat.com>


> A tested source reproducer is available privately on request.  No
> controlled read or write, information leak, or privilege escalation is
> claimed.  The mainline patch applies directly to v6.19.y and newer;
> v6.16.y through v6.18.y need a context-adjusted backport.

Up to you and Christian (I am hoping we can take your fix), but I think
that this paragraph should be removed from the changelog... At least the
"A tested source reproducer is available privately on request." part.

Heh. I didn't even know that 0a36bad01731 was backported to stable. Why?
I guess I missed some AUTOSEL email...

Thanks,

Oleg.