linux-kernel - Re: [PATCH 5/6] proc: use task_access_lock() instead of ptrace_may

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Date:	Thu, 12 Apr 2012 16:29:27 +0100
From:	Djalal Harouni <tixxdz@...ndz.org>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH 5/6] proc: use task_access_lock() instead of
 ptrace_may_access()

Hi Cong,

On Thu, Apr 12, 2012 at 09:22:10PM +0800, Cong Wang wrote:
> On 04/11/2012 01:59 PM, Cong Wang wrote:
> > There are several places in fs/proc/base.c still use ptrace_may_access()
> > directly to check the permission, actually this just gets a snapshot of
> > the permission, nothing prevents the target task from raising the priviledges
> > itself, it is better to use task_access_lock() for these places, to hold
> > the priviledges.
> >
> Hi, Andrew,
> 
> Please drop this patch, it introduces a deadlock when execve() a 
> /proc/<pid>/exec file, and it is not a big improvement nor fixes any 
> bugs, so let's just drop this one.
I was going to ask about this since it seems that abusing lock_trace()
or task_access_lock() can cause problems.

Please see commit 5e442a493fc59f which reverts commit aa6afca5bcaba8101f
that tries to protect /proc/PID/fd** files


IMHO A solution for some of the simple /proc/<pid>/* files is to use
mm_access() check just after gathering data and before returning it to
userspace.

So IMO the original code of proc_pid_wchan() was correct, since that data
is not copied to userspace directly, and we can avoid the mm_access() and
the task->signal->cred_guard_mutex lock since we do not race against them,
we have already grabbed the 'wchan', a simple ptrace_may_access() check will
do the job.

(I guess there is a window against another execve and ptrace_may_access()
but that returned data is not useful anymore, is it ?).


For others I don't know what would be the best solution.

Thanks.

> Thanks!
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