linux-kernel - Re: [PATCH] mm: avoid dirtying shared mappings on mlock

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 12 Oct 2007 12:50:22 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Suleiman Souhlal <ssouhlal@...ebsd.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Suleiman Souhlal <suleiman@...gle.com>,
	linux-mm <linux-mm@...ck.org>, hugh <hugh@...itas.com>
Subject: Re: [PATCH] mm: avoid dirtying shared mappings on mlock

On Fri, 2007-10-12 at 04:14 +1000, Nick Piggin wrote:
> On Friday 12 October 2007 20:37, Peter Zijlstra wrote:
> > On Fri, 2007-10-12 at 02:57 +1000, Nick Piggin wrote:
> > > On Friday 12 October 2007 19:03, Peter Zijlstra wrote:
> > > > Subject: mm: avoid dirtying shared mappings on mlock
> > > >
> > > > Suleiman noticed that shared mappings get dirtied when mlocked.
> > > > Avoid this by teaching make_pages_present about this case.
> > > >
> > > > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > > > Acked-by: Suleiman Souhlal <suleiman@...gle.com>
> > >
> > > Umm, I don't see the other piece of this thread, so I don't
> > > know what the actual problem was.
> > >
> > > But I would really rather not do this. If you do this, then you
> > > now can get random SIGBUSes when you write into the memory if it
> > > can't allocate blocks or ... (some other filesystem specific
> > > condition).
> >
> > I'm not getting this, make_pages_present() only has to ensure all the
> > pages are read from disk and in memory. How is this different from a
> > read-scan?
> 
> I guess because we've mlocked a region that has PROT_WRITE access...
> but actually, I suppose mlock doesn't technically require that we
> can write to the memory, only that the page isn't swapped out.
> 
> Still, it is nice to be able to have a reasonable guarantee of
> writability.
> 
> 
> > The pages will still be read-only due to dirty tracking, so the first
> > write will still do page_mkwrite().
> 
> Which can SIGBUS, no?

Sure, but that is no different than any other mmap'ed write. I'm not
seeing how an mlocked region is special here.

I agree it would be nice if mmap'ed writes would have better error
reporting than SIGBUS, but such is life.

Download attachment "signature.asc" of type "application/pgp-signature" (190 bytes)