linux-kernel - Re: [PATCH] mm: prevent mmap_cache race in find

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:	Thu, 4 Apr 2013 12:01:52 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Stancek <jstancek@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ian Lance Taylor <iant@...gle.com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: prevent mmap_cache race in find_vma()

On Thu, 4 Apr 2013, Linus Torvalds wrote:
> On Thu, Apr 4, 2013 at 11:35 AM, Hugh Dickins <hughd@...gle.com> wrote:
> >
> > find_vma() can be called by multiple threads with read lock
> > held on mm->mmap_sem and any of them can update mm->mmap_cache.
> > Prevent compiler from re-fetching mm->mmap_cache, because other
> > readers could update it in the meantime:
> 
> Ack. I do wonder if we should mark the unlocked update too some way
> (also in find_vma()), although it's probably not a problem in practice
> since there's no way the compiler can reasonably really do anything
> odd with it. We *could* make that an ACCESS_ONCE() write too just to
> highlight the fact that it's an unlocked write to this optimistic data
> structure.

Hah, you beat me to it.

I wanted to get Jan's patch in first, seeing as it actually fixes his
observed issue; and it is very nice to have such a good description of
one of those, when ACCESS_ONCE() is usually just an insurance policy.

But then I was researching the much rarer "ACCESS_ONCE(x) = y" usage
(popular in drivers/net/wireless/ath/ath9k and kernel/rcutree* and
sound/firewire, but few places else).

When Paul reminded us of it yesterday, I came to wonder if actually
every use of ACCESS_ONCE in the read form should strictly be matched
by ACCESS_ONCE whenever modifying the location.

My uneducated guess is that strictly it ought to, in the sense of
insurance policy; but that (apart from that strange split writing
issue which came up a couple of months ago) in practice our compilers
have not "advanced" to the point of making this an issue yet.

> 
> Anyway, applied.

Thanks,
Hugh
--
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/