linux-kernel - Re: [PATCH v5 3/6] seccomp: introduce writer locking

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Date:	Fri, 23 May 2014 10:49:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	James Morris <james.l.morris@...cle.com>,
	Eric Paris <eparis@...hat.com>,
	Juri Lelli <juri.lelli@...il.com>,
	John Stultz <john.stultz@...aro.org>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Alex Thorlton <athorlton@....com>,
	Rik van Riel <riel@...hat.com>,
	Daeseok Youn <daeseok.youn@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Dario Faggioli <raistlin@...ux.it>,
	Rashika Kheria <rashika.kheria@...il.com>,
	liguang <lig.fnst@...fujitsu.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	linux-doc@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH v5 3/6] seccomp: introduce writer locking

On Thu, May 22, 2014 at 04:05:33PM -0700, Kees Cook wrote:
> Normally, task_struct.seccomp.filter is only ever read or modified by
> the task that owns it (current). This property aids in fast access
> during system call filtering as read access is lockless.
> 
> Updating the pointer from another task, however, opens up race
> conditions. To allow cross-task filter pointer updates, writes to the
> seccomp fields are now protected by a spinlock.  Read access remains
> lockless because pointer updates themselves are atomic.  However, writes
> (or cloning) often entail additional checking (like maximum instruction
> counts) which require locking to perform safely.
> 
> In the case of cloning threads, the child is invisible to the system
> until it enters the task list. To make sure a child can't be cloned
> from a thread and left in a prior state, seccomp duplication is moved
> under the tasklist_lock. Then parent and child are certain have the same
> seccomp state when they exit the lock.
> 

So I'm a complete noob on the whole seccomp thing, so maybe this is a
silly question, but.. what about object lifetimes?

Looking at put_seccomp_filter() it explicitly takes a tsk pointer,
suggesting one can call it on !current. And while it does a dec_and_test
on the object itself, run_filter() does nothing with refcounts, and
therefore can be touching dead memory.



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