linux-kernel - Re: How to temporary change 'current' (task)

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Message-ID: <20110617095625.GA7627@albatros>
Date:	Fri, 17 Jun 2011 13:56:25 +0400
From:	Vasiliy Kulikov <segoon@...nwall.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	kernel-hardening@...ts.openwall.com
Subject: Re: How to temporary change 'current' (task)

On Fri, Jun 17, 2011 at 11:29 +0200, Peter Zijlstra wrote:
> On Fri, 2011-06-17 at 12:36 +0400, Vasiliy Kulikov wrote:
> > I wonder whether there is a simple way to temporary switch 'current' to
> > another task and then switch it back with minimum side effects? 
> 
> No.

After a moment's thought, it would also break some synchronization that
relies on some structures are accesses only from the current task.

> >  I need
> > it to call "reversed" ptrace_may_access() with swapped current and
> > target task.  Introducing ptrace_task_may_access_me() would produce too
> > much noise in LSM (it also needs reversed security_ptrace_access_check()),
> > which is too loud for my needs.
> > 
> > Specifically, I need it to filter taskstats and proc connector requests
> > for a restriction of getting other processes' information:
> > 
> > http://permalink.gmane.org/gmane.linux.kernel/1155354
> > 
> > As the check is handled in the context of the ptrace target process,
> > ptrace_may_access() doesn't fit my needs.
> 
> looking at __ptrace_may_access() it doesn't look too hard to make it
> take two task arguments and use __task_cred() twice instead of
> current_cred().
> 
> It of course needs extending security_ptrace_access_check() as well, but
> that comes with the territory.

Well, yes, but it implies pushing explicit ptrace actor into LSM
modules.  OK, it should be not as noisy as I initially thought - almost
all current LSMs already use "current" as an explicit argument in
theirs internal functions.

Thank you,

-- 
Vasiliy
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