linux-kernel - Re: [PATCH 2/2] sys, seccomp: add PR_SECCOMP_EXT and SECCOMP_EXT_ACT

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Message-ID: <20140114192118.GA31411@redhat.com>
Date:	Tue, 14 Jan 2014 20:21:18 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Will Drewry <wad@...omium.org>
Cc:	linux-kernel@...r.kernel.org, nschichan@...ebox.fr,
	keescook@...omium.org, james.l.morris@...cle.com,
	akpm@...ux-foundation.org, holt@....com, viro@...iv.linux.org.uk
Subject: Re: [PATCH 2/2] sys, seccomp: add PR_SECCOMP_EXT and
	SECCOMP_EXT_ACT_TSYNC

On 01/13, Will Drewry wrote:
>
> +static pid_t seccomp_sync_threads(void)
> +{
> +	struct task_struct *thread, *caller;
> +	pid_t failed = 0;
> +	thread = caller = current;
> +
> +	read_lock(&tasklist_lock);
> +	if (thread_group_empty(caller))
> +		goto done;
> +	while_each_thread(caller, thread) {
> +		task_lock(thread);

perhaps we take task_lock() to serialize with another caller of
seccomp_sync_threads()...

If yes, then perhaps you can use ->siglock instead of tasklist_lock
and do not use task_lock(). It would be even better to rely on rcu,
but:

> +			get_seccomp_filter(caller);
> +			/*
> +			 * Drop the task reference to the shared ancestor since
> +			 * current's path will hold a reference.  (This also
> +			 * allows a put before the assignment.)
> +			 */
> +			put_seccomp_filter(thread);
> +			thread->seccomp.filter = caller->seccomp.filter;

As I said, I do not understand this patch yet, but this looks suspicious.

Why we can't race with this thread doing clone(CLONE_THREAD) ? We do
not the the new thread yet, but its ->seccomp can be already copied
by copy_process(), no?

Oleg.

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