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

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Message-ID: <20120428142605.GA20248@redhat.com>
Date:	Sat, 28 Apr 2012 16:26:05 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Mike Galbraith <efault@....de>
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 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()

> 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.

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.

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?

Oleg.

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