linux-kernel - Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

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Date:   Thu, 1 Nov 2018 13:56:35 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian@...uner.io>,
        Tyler Hicks <tyhicks@...onical.com>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>,
        Aleksa Sarai <asarai@...e.de>, linux-kernel@...r.kernel.org,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

On Thu, Nov 01, 2018 at 02:40:02PM +0100, Oleg Nesterov wrote:
> On 10/29, Tycho Andersen wrote:
> >
> > +static struct file *init_listener(struct seccomp_filter *filter)
> > +{
> > +	struct file *ret = ERR_PTR(-EBUSY);
> > +	struct seccomp_filter *cur, *last_locked = NULL;
> > +	int filter_nesting = 0;
> > +
> > +	for (cur = current->seccomp.filter; cur; cur = cur->prev) {
> > +		mutex_lock_nested(&cur->notify_lock, filter_nesting);
> > +		filter_nesting++;
> > +		last_locked = cur;
> > +		if (cur->notif)
> > +			goto out;
> > +	}
> 
> Somehow I no longer understand why do you need to take all locks. Isn't
> the first filter's notify_lock enough? IOW,
> 
> 		for (cur = current->seccomp.filter; cur; cur = cur->prev) {
> 			if (cur->notif)
> 				return ERR_PTR(-EBUSY);
> 			first = cur;
> 		}
> 
> 		if (first)
> 			mutex_lock(&first->notify_lock);
> 
> 		... initialize filter->notif ...
> 
> 	out:
> 		if (first)
> 			mutex_unlock(&first->notify_lock);
> 
> 		return ret;

The idea here is to prevent people from "nesting" notify filters. So
if any filter in the chain has a listener attached, it refuses to
install another filter with a listener.

But it just occurred to me that we don't handle the TSYNC case
correctly by doing it this way, and it's not necessarily obvious to me
how we can :). So let me look into that.

Tycho