linux-kernel - Re: [PATCH v2 3/3] tty: Use RCU read lock to iterate tasks and threads in __do

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Date:   Wed, 17 Jan 2018 10:54:00 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     gregkh@...uxfoundation.org, jslaby@...e.com, oleg@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] tty: Use RCU read lock to iterate tasks and threads in __do_SAK()

Kirill Tkhai <ktkhai@...tuozzo.com> writes:

> There were made several efforts to make __do_SAK()
> working in process context long ago, but it does
> not solves the problem completely. Since __do_SAK()
> may take tasklist_lock for a long time, the concurent
> processes, waiting for write lock with interrupts
> disabled (e.g., forking), get into the same situation
> like __do_SAK() would have been executed in interrupt
> context. I've observed several hard lockups on 3.10
> kernel running 200 containers, caused by long duration
> of copy_process()->write_lock_irq() after SAK was sent
> to a tty. Current mainline kernel has the same problem.
>
> The solution is to use RCU to iterate processes and threads.
> Task list integrity is the only reason we taken tasklist_lock
> before, as tty subsys primitives mostly take it for reading
> also (e.g., __proc_set_tty). RCU read lock is enough for that.
> This patch solves the problem and makes __do_SAK() to be
> not greedy of tasklist_lock. That should prevent hard lockups
> I've pointed above.

__do_SAK() needs to be 100% accurate.  I do not see the rcu_read_lock
guaranteeing that new processes created while the process list is being
iterated that happen to have a reference to the tty will be seen.

So I do not believe this is the actual fix to the problem.  Especially
not if we intend to for SAK to remain a secure attention key that
guarantees no other processes have access to the tty.

Eric


> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
> ---
>  drivers/tty/tty_io.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 89326cee2403..55115e65668d 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -2724,7 +2724,9 @@ void __do_SAK(struct tty_struct *tty)
>  			   task_pid_nr(p), p->comm);
>  		send_sig(SIGKILL, p, 1);
>  	} while_each_pid_task(session, PIDTYPE_SID, p);
> +	read_unlock(&tasklist_lock);
>  
> +	rcu_read_lock();
>  	/* Now kill any processes that happen to have the tty open */
>  	for_each_process(p) {
>  		if (p->signal->tty == tty) {
> @@ -2754,7 +2756,7 @@ void __do_SAK(struct tty_struct *tty)
>  kill:
>  		send_sig(SIGKILL, p, 1);
>  	}
> -	read_unlock(&tasklist_lock);
> +	rcu_read_unlock();
>  #endif
>  }
>