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

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Date:	Tue, 14 Jan 2014 21:13:11 +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 01/14, Oleg Nesterov wrote:
>
> On 01/14, Oleg Nesterov wrote:
> >
> > > +			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?
>
> And it seems that this can obviously race with seccomp_attach_filter()
> called by this "thread".

Heh. I just noticed that this patch is not first in series, and I wasn't
cc'ed. I found this one on marc.info,

	http://marc.info/?l=linux-kernel&m=138964557211277

this explains task_lock(). But this can't fix the race with copy_process,
and the patch itself doesn't look right... if nothing else, we can't do
copy_from_user() under task_lock().

Oleg.

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