linux-kernel - Re: [PATCH -next v2 0/4] mm: replace mmap_sem for mm->exe

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Message-ID: <20150320160910.GA27066@moon>
Date:	Fri, 20 Mar 2015 19:09:10 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Davidlohr Bueso <dave@...olabs.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>, koct9i@...il.com,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next v2 0/4] mm: replace mmap_sem for mm->exe_file
 serialization

On Mon, Mar 16, 2015 at 03:08:40PM -0700, Kees Cook wrote:
> >
> >> Ok I think I am finally seeing where you are going. And I like it *a
> >> lot* because it allows us to basically replace mmap_sem with rcu
> >> (MMF_EXE_FILE_CHANGED being the only user that requires a lock!!), but
> >> am afraid it might not be possible. I mean currently we have no rule wrt
> >> to users that don't deal with prctl.
> >>
> >> Forbidding multiple exe_file changes to be generic would certainly
> >> change address space semantics, probably for the better (tighter around
> >> security), but changed nonetheless so users would have a right to
> >> complain, no? So if we can get away with removing MMF_EXE_FILE_CHANGED
> >> I'm all for it. Andrew?
> 
> I can't figure out why MMF_EXE_FILE_CHANGED is used to stop a second
> change. But it does seem useful to mark a process as "hey, we know for
> sure this the exe_file changed on this process" from an accounting
> perspective.

Sure, except it start being more stopper for further development so
ripping it off would help ;)

> 
> And I'd agree about the malware: it would never use this interface, so
> there's no security benefit I can see. Maybe I haven't had enough
> coffee, though. :)

Yes, same here, would never use it either.
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