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Date: Wed, 9 May 2012 21:30:55 +0200
From: Petr Matousek <>
Subject: Re: CVE Request -- kernel: futex: clear robust_list
 on execve

On Tue, May 08, 2012 at 04:31:53AM +0400, Solar Designer wrote:
> On Tue, May 08, 2012 at 04:08:17AM +0400, Solar Designer wrote:
> > Indeed, execve() may make the new process relatively privileged (SUID,
> > SGID, fscaps), and thus being able to write into its memory is a
> > security issue.  However, it appears that robust_list (and its compat
> > counterpart) is only used for such writes when the process itself is
> > exiting (with the aim being to notify other threads sharing the same
> > mm).  If so, the question is whether and how writes into an exiting
> > process' memory may be exploited.  We're already in do_exit() at this
> > point, and it's just a few lines before we detach from and likely
> > destroy the mm.  Well, if that process itself is multi-threaded (and
> > other threads are not exiting yet), it possibly can be exploited
> > (through affecting those other threads).
> describes that the
> bug was inadvertently triggered in normal usage of certain programs, and
> how it was rather difficult to figure out.  My question is: was exit of
> a multi-threaded program involved and relevant?  If not, then there must
> be something wrong with my reasoning, because I don't currently see how
> the bug may otherwise have visible consequences.

In this case single-threaded (privileged) Xorg was run with a stale
robust list pointer that accidentally fell into MMIO area (see how
ioperm() is implemnted on IA64). Because of the way ill-sized MMIO
requests (exit_robust_list()) were handled in qemu-dm, the result was
guest crash (all of this happened in Xen guest).

Petr Matousek / Red Hat Security Response Team

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