linux-kernel - Re: [PATCH 3/3] ptrace: do not use task

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Date:	Fri, 08 May 2009 13:18:20 +0100
From:	David Howells <dhowells@...hat.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	dhowells@...hat.com, James Morris <jmorris@...ei.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	Eric Paris <eparis@...hat.com>,
	Stephen Smalley <sds@...ho.nsa.gov>
Subject: Re: [PATCH 3/3] ptrace: do not use task_lock() for attach

Roland McGrath <roland@...hat.com> wrote:

> Good catch, Chris and Oleg!  This one is yet another dhowells blue plate
> special, deeply subtle change buried inside the ginormous commit d84f4f9. ;-}

Well...  Unfortunately it was an all-or-nothing patch, leastways if you wanted
the kernel to compile afterwards.

> He even mentioned this one in the log:
> 
>  (a) selinux_setprocattr() no longer does its check for whether the
> 	 current ptracer can access processes with the new SID inside the lock
> 	 that covers getting the ptracer's SID.  Whilst this lock ensures that
> 	 the check is done with the ptracer pinned, the result is only valid
> 	 until the lock is released, so there's no point doing it inside the
> 	 lock.

I knew there was a reason I carefully documented the major changes.

> Before d84f4f9, the extraction, avc check, and SID switch were all under
> task_lock().  What David's comment ignores is that "the lock that covers
> getting the ptracer's SID" (i.e. task_lock) is also the lock that excludes
> ptrace attempts, with their security checks against the (old or new) SID.
> i.e.:

I mainly focused on making sure ptrace and execve still worked in relation to
each other.  Unfortunately, I didn't see that selinux_setprocattr() might
interact with ptrace() in the same manner.

> Indeed, cred_exec_mutex is the equivalent lock for that post-d84f4f9.

Yeah.  Perhaps it should be renamed cred_ptrace_mutex.

David
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