linux-kernel - Re: [PATCH] [BUGFIX] update mm->owner even if no next owner.

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]

Message-ID: <20110611180415.GB31154@cmpxchg.org>
Date:	Sat, 11 Jun 2011 20:04:15 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com>,
	Ying Han <yinghan@...gle.com>, Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] [BUGFIX] update mm->owner even if no next owner.

On Sat, Jun 11, 2011 at 06:39:43PM +0200, Andrea Arcangeli wrote:
> On Sat, Jun 11, 2011 at 09:04:14AM -0700, Hugh Dickins wrote:
> > I had another go at reproducing it, 2 hours that time, then a try with
> > 692e0b35427a reverted: it ran overnight for 9 hours when I stopped it.
> > 
> > Andrea, please would you ask Linus to revert that commit before -rc3?
> > Or is there something else you'd like us to try instead?  I admit that
> > I've not actually taken the time to think through exactly how it goes
> > wrong, but it does look dangerous.
> 
> Here I was asked if the mem_cgroup_newpage_charge need the mmap_sem at
> all. And if not why not to release the mmap_sem early.
> 
> https://lkml.org/lkml/2011/3/14/276
> 
> So I didn't see why mmap_sem was needed, I also asked confirmation and
> who answered agreed it was safe without mmap_sem even if it's the only
> place doing that. Maybe that assumption was wrong and we need
> mmap_sem after all if this commit is causing problems.
> 
> Or did you find something wrong in the actual patch?
> 
> Do I understand right that the bug just that we must run
> alloc_hugepage_vma+mem_cgroup_newpage_charge within the same critical
> section protected by the mmap_sem read mode? Do we know why?

The problem is that mm->owner points to a stale task structure if the
last possible owner is exiting.  The mmap_sem just prevented the task
from actually exiting through write-acquiring the mmap_sem in
khugepaged_exit().

I think enforcing lifetime of an object through locks is not the
nicest thing to do, so I stand by what I wrote in the mail you linked
to above :) and agree with Kame that mm->owner should just not point
to a stale task struct.  The memcg code can handle it going NULL.

--
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/