linux-kernel - Re: [RFC/PATCH] mm/futex: Fix futex writes on archs with SW tracking of dirty & young

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, 22 Jul 2011 08:52:06 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Shan Hai <haishan.bai@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, paulus@...ba.org,
	tglx@...utronix.de, walken@...gle.com, dhowells@...hat.com,
	cmetcalf@...era.com, tony.luck@...el.com,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC/PATCH] mm/futex: Fix futex writes on archs with SW
 tracking of dirty & young

On Thu, 2011-07-21 at 15:36 -0700, Andrew Morton wrote:
> On Tue, 19 Jul 2011 14:29:22 +1000
> Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:
> 
> > The futex code currently attempts to write to user memory within
> > a pagefault disabled section, and if that fails, tries to fix it
> > up using get_user_pages().
> > 
> > This doesn't work on archs where the dirty and young bits are
> > maintained by software, since they will gate access permission
> > in the TLB, and will not be updated by gup().
> > 
> > In addition, there's an expectation on some archs that a
> > spurious write fault triggers a local TLB flush, and that is
> > missing from the picture as well.
> > 
> > I decided that adding those "features" to gup() would be too much
> > for this already too complex function, and instead added a new
> > simpler fixup_user_fault() which is essentially a wrapper around
> > handle_mm_fault() which the futex code can call.
> > 
> > Signed-off-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> > ---
> > 
> > Shan, can you test this ? It might not fix the problem
> 
> um, what problem.  There's no description here of the user-visible
> effects of the bug hence it's hard to work out what kernel version(s)
> should receive this patch.

Shan could give you an actual example (it was in the previous thread),
but basically, livelock as the kernel keeps trying and trying the
in_atomic op and never resolves it.
 
> What kernel version(s) should receive this patch?

I haven't dug. Probably anything it applies on as far as we did that
trick of atomic + gup() for futex.

> > since I'm
> > starting to have the nasty feeling that you are hitting what is
> > somewhat a subtly different issue or my previous patch should
> > have worked (but then I might have done a stupid mistake as well)
> > but let us know anyway.
> 
> I assume that Shan reported the secret problem so I added the
> reported-by to the changelog.

He did :-) Shan, care to provide a rough explanation of what you
observed ?

Also Russell confirmed that ARM should be affected as well.

Cheers,
Ben.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/