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

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Message-ID: <6599ad830902100215o185edd31k8731e033717dbb49@mail.gmail.com>
Date:	Tue, 10 Feb 2009 02:15:36 -0800
From:	Paul Menage <menage@...gle.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	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 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().

If anyone can call cgroup_test_super() on the sb that we're trying to
free, isn't that bad? Since even with this patch we free root just
after calling kill_litter_sb(), if anyone tries to reuse that super
block then they're toast.

Would it be better to set sb->s_fs_info to NULL, before calling
kill_litter_sb(), and have cgroup_test_super() just return 0 for any
sb with a NULL sb->s_fs_info.

Paul

>
> Reported-by: Al Viro <viro@...IV.linux.org.uk>
> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
> ---
>  kernel/cgroup.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 5a54ff4..42fd3f2 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1122,8 +1122,8 @@ static void cgroup_kill_sb(struct super_block *sb) {
>
>        mutex_unlock(&cgroup_mutex);
>
> -       kfree(root);
>        kill_litter_super(sb);
> +       kfree(root);
>  }
>
>  static struct file_system_type cgroup_fs_type = {
> --
> 1.5.4.rc3
>
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