Re: strange interaction between fuse + pidns

["Eric W. Biederman" <ebiederm@xmission.com>]

Tycho Andersen <tycho@tycho.pizza> writes:

> Hi all,
>
> On Mon, Jul 11, 2022 at 03:59:15PM +0200, Miklos Szeredi wrote:
>> On Mon, 11 Jul 2022 at 12:35, Miklos Szeredi <miklos@szeredi.hu> wrote:
>> >
>> > Can you try the attached untested patch?
>> 
>> Updated patch to avoid use after free on req->args.
>> 
>> Still mostly untested.
>
> Thanks, when I applied your patch, I still ended up with tasks stuck
> waiting with a SIGKILL pending. So I looked into that and came up with
> the patch below. With both your patch and mine, my testcase exits
> cleanly.
>
> Eric (or Christian, or anyone), can you comment on the patch below? I
> have no idea what this will break. Maybe instead a better approach is
> some additional special case in __send_signal_locked()?
>
> Tycho
>
> From b7ea26adcf3546be5745063cc86658acb5ed37e9 Mon Sep 17 00:00:00 2001
> From: Tycho Andersen <tycho@tycho.pizza>
> Date: Mon, 11 Jul 2022 11:26:58 -0600
> Subject: [PATCH] sched: __fatal_signal_pending() should also check shared
>  signals
>
> The wait_* code uses signal_pending_state() to test whether a thread has
> been interrupted, which ultimately uses __fatal_signal_pending() to detect
> if there is a fatal signal.
>
> When a pid ns dies, in zap_pid_ns_processes() it does:
>
>     group_send_sig_info(SIGKILL, SEND_SIG_PRIV, task, PIDTYPE_MAX);
>
> for all the tasks in the pid ns. That calls through:
>
>     group_send_sig_info() ->
>       do_send_sig_info() ->
>         send_signal_locked() ->
>           __send_signal_locked()
>
> which does:
>
>     pending = (type != PIDTYPE_PID) ? &t->signal->shared_pending : &t->pending;
>
> which puts sigkill in the set of shared signals, but not the individual
> pending ones. If tasks are stuck in a killable wait (e.g. a fuse flush
> operation), they won't see this shared signal, and will hang forever, since
> TIF_SIGPENDING is set, but the fatal signal can't be detected.

Hmm.

That is perplexing.

__send_signal_locked calls complete_signal.  Then if any of the tasks of
the process can receive the signal, complete_signal will loop through
all of the tasks of the process and set the per thread SIGKILL.  Pretty
much by definition tasks can always receive SIGKILL.

Is complete_signal not being able to do that?

The patch below really should not be necessary, and I have pending work
that if I can push over the finish line won't even make sense.

As it is currently an abuse to use the per thread SIGKILL to indicate
that a fatal signal has been short circuit delivered.  That abuse as
well as being unclean tends to confuse people reading the code.

Eric

> Signed-off-by: Tycho Andersen <tycho@tycho.pizza>
> ---
>  include/linux/sched/signal.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index cafbe03eed01..a033ccb0a729 100644
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -402,7 +402,8 @@ static inline int signal_pending(struct task_struct *p)
>  
>  static inline int __fatal_signal_pending(struct task_struct *p)
>  {
> -	return unlikely(sigismember(&p->pending.signal, SIGKILL));
> +	return unlikely(sigismember(&p->pending.signal, SIGKILL) ||
> +			sigismember(&p->signal->shared_pending.signal, SIGKILL));
>  }
>  
>  static inline int fatal_signal_pending(struct task_struct *p)
>
> base-commit: 32346491ddf24599decca06190ebca03ff9de7f8