linux-kernel - Re: + signal-always-clear-sa_restorer-on-execve.patch added to -mm tree

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Message-Id: <20130311142218.272033794123531b119ca68e@linux-foundation.org>
Date:	Mon, 11 Mar 2013 14:22:18 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	Ben Hutchings <ben@...adent.org.uk>,
	luis.henriques@...onical.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: + signal-always-clear-sa_restorer-on-execve.patch added to -mm
 tree

On Mon, 11 Mar 2013 14:03:20 -0700 Kees Cook <keescook@...omium.org> wrote:

> On Mon, Mar 11, 2013 at 2:01 PM, Andrew Morton
> <akpm@...ux-foundation.org> wrote:
> > On Mon, 11 Mar 2013 13:37:53 -0700 Kees Cook <keescook@...omium.org> wrote:
> >
> >> ...
> >>
> >
> > (pop toasting undone)
> >
> >> > Subject: signal: always clear sa_restorer on execve
> >> >
> >> > When the new signal handlers are set up, the location of sa_restorer is
> >> > not cleared, leaking a parent process's address space location to
> >> > children.  This allows for a potential bypass of the parent's ASLR by
> >> > examining the sa_restorer value returned when calling sigaction().
> >> >
> >> > Based on what should be considered "secret" about addresses, it only
> >> > matters across the exec not the fork (since the VMAs haven't changed until
> >> > the exec).  But since exec sets SIG_DFL and keeps sa_restorer, this is
> >> > where it should be fixed.
> >>
> >> A note for backporters: you'll likely want to change
> >> __ARCH_HAS_SA_RESTORER to SA_RESTORER, since the former was recently
> >> introduced. If not, this will apply but not actually do any good.
> >
> > I added this to the changelog, but I fear people won't read it!  Is
> > there any clever way in which we can have one patch which will work OK
> > in both old and new kernels?  I can't think of one...
> 
> Using just SA_RESTORER will work in both cases, but isn't "correct"
> going forward. :(

That's easy.

patch #1: use SA_RESTORER, cc stable (please promise me this will work OK)
patch #2: switch to __ARCH_HAS_SA_RESTORER, no cc stable

I'm assuming this is all for 3.10, btw.  If you think it should be in
3.9 then you need to write scarier changelogs.

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