linux-kernel - Re: [PATCH] cgroups: fix possible use after free

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Message-ID: <6599ad830902101601i294ffaa5xd01611c5121a5685@mail.gmail.com>
Date:	Tue, 10 Feb 2009 16:01:07 -0800
From:	Paul Menage <menage@...gle.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Li Zefan <lizf@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: [PATCH] cgroups: fix possible use after free

On Tue, Feb 10, 2009 at 4:45 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Tue, Feb 10, 2009 at 02:15:36AM -0800, Paul Menage wrote:
>> On Tue, Feb 10, 2009 at 1:31 AM, Li Zefan <lizf@...fujitsu.com> wrote:
>> > In cgroup_kill_sb(), root is freed before sb is detached from the list,
>> > so another sget() may find this sb and call cgroup_test_super(),
>> > which will access the root that has been freed.
>>
>> I think that I'd assumed that by the time we get to cgroup_kill_sb()
>> there's no chance of the sb being resurrected by sget().
>
> There is none.  grab_super() will fail to get it, so sget() will go
> through retry logics.  Which doesn't mean that test won't be called
> on it in the meanwhile.

OK, so Zefan's patch looks like the safest way to fix this particular
issue. I think I see some other potential races with
cgroup_test_super() though - we probably need to synchronize against
the changing of a root's subsys_bits in rebind_subsystems(). Taking
cgroup_mutex around the call to sget() would certainly provide that,
but I'd have to check whether it causes locking cycles.

Paul
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