linux-kernel - Re: [RFC PATCH] namespaces: fix leak on fork() failure

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Message-ID: <1335672824.7366.56.camel@marge.simpson.net>
Date:	Sun, 29 Apr 2012 06:13:44 +0200
From:	Mike Galbraith <efault@....de>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Louis Rilling <louis.rilling@...labs.com>
Subject: Re: [RFC PATCH] namespaces: fix leak on fork() failure

On Sat, 2012-04-28 at 16:26 +0200, Oleg Nesterov wrote: 
> On 04/28, Mike Galbraith wrote:
> >
> > Greetings,
> 
> Hi,
> 
> Add CC's. I never understood the proc/namespace interaction in details,
> and it seems to me I forgot everything.
> 
> > SIGCHLD delivery during fork() may cause failure,
> 
> Or any other reason to fail after copy_namespaces()

Yeah.

> > resulting in the aborted
> > child being cloned with CLONE_NEWPID leaking namespaces due to proc being
> > mounted during pid namespace creation, but not unmounted on fork() failure.
> 
> Heh. Please look at http://marc.info/?l=linux-kernel&m=127687751003902
> and the whole thread, there are a lot more problems here.

Ew, I would have been better off not reading that ;-)

> But this particular one looks simple iirc.
> 
> > @@ -216,6 +216,14 @@ void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
> >  	rcu_assign_pointer(p->nsproxy, new);
> >
> >  	if (ns && atomic_dec_and_test(&ns->count)) {
> > +		/* Handle fork() failure, unmount proc before proceeding */
> > +		if (unlikely(!new && !((p->flags & PF_EXITING)))) {
> > +			struct pid_namespace *pid_ns = ns->pid_ns;
> > +
> > +			if (pid_ns && pid_ns != &init_pid_ns)
> > +				pid_ns_release_proc(pid_ns);
> > +		}
> > +
> >  		/*
> >  		 * wait for others to get what they want from this nsproxy.
> >  		 *
> 
> At first glance this looks correct. But the PF_EXITING check doesn't
> look very nice imho. It is needed to detect the case when the caller
> is copy_process()->bad_fork_cleanup_namespaces and p is not current.

Yeah, that does look a lot like a wart.

This being the first use of pid_ns_release_proc(), I was more concerned
that perhaps I should be doing something else entirely.

> Perhaps it would be more clean to add the explicit
> 
> 	 bad_fork_cleanup_namespaces:
> 	+	if (unlikely(clone_flags & CLONE_NEWPID))
> 	+		pid_ns_release_proc(...);
> 		exit_task_namespaces(p);
> 		
> 		
> code into this error path in copy_process?

Yeah, that's prettier.  Thanks.

-Mike

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