[PATCH] cgroups: fix possible use after free

[Paul Menage]

On Tue, Feb 10, 2009 at 4:45 AM, Al Viro <viro at zeniv.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 at cn.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