linux-kernel - Re: [PATCH 3/5] device_cgroup: keep track of local group settings

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Date:	Thu, 29 Nov 2012 14:59:42 -0500
From:	Aristeu Rozanski <aris@...hat.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	cgroups@...r.kernel.org
Subject: Re: [PATCH 3/5] device_cgroup: keep track of local group settings

On Thu, Nov 29, 2012 at 07:29:45PM +0000, Serge E. Hallyn wrote:
> Quoting Aristeu Rozanski (aris@...hat.com):
> > In preparation for better hierarchy support, it's needed to retain the local
> > settings in order to try to reapply them after a propagated change if they're
> > still valid.
> > 
> > Cc: Tejun Heo <tj@...nel.org>
> > Cc: Serge Hallyn <serge.hallyn@...onical.com>
> > Signed-off-by: Aristeu Rozanski <aris@...hat.com>
> >  
> > ---
> >  security/device_cgroup.c |  108 ++++++++++++++++++++++++++++++++++-------------
> >  1 file changed, 80 insertions(+), 28 deletions(-)
> 
> Hi,
> 
> thanks for doing this.  I've got one concern though.  I don't see
> any place where devcgroup_create() was updated to create the
> local exceptions list.  I think we need a guarantee that at

the local exceptions list is part of the dev_cgroup structure and it's
initialized on the patch 'device_cgroup: keep track of local group
setting':

@@ -190,6 +238,8 @@
        if (!dev_cgroup)
                return ERR_PTR(-ENOMEM);
        INIT_LIST_HEAD(&dev_cgroup->exceptions);
+       INIT_LIST_HEAD(&dev_cgroup->local.exceptions);
+       dev_cgroup->local.behavior = DEVCG_DEFAULT_NONE;
        parent_cgroup = cgroup->parent;

        if (parent_cgroup == NULL)

> any time the local exceptions list will contain all the entries
> contained in the cgroup->exceptions list.  Otherwise you cannot

hm, no. the local exceptions are meant to be used for stuff written
locally. the devcgroup->exceptions list is the list in effect.

> use the exception_add() the way you do (or you can't use RCU),
> since there could be a window between the successful addition
> of a rule to cgroup->local.exceptions and the failed addition to
> cgroup->exceptions, during which a task (since it won't need
> the mutex for devcg_allow()) could exceed allowed permissions.
>
> It's possible I'm misunderstanding.  If you think that's the case
> just kick me and I'll take a fresh look.

I see your point. it's indeed a problem. in dev_exception_add(), it
needs to check for permissions before actually adding to
devcgroup->exceptions.

> (Btw is there a git tree or gitweb view I could look at alongside
> the patchset?)

I'll rebase the patchset along with a fix for this and resubmit with a
link to the git repo.

-- 
Aristeu

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