Messages in this thread | | | Date | Tue, 10 Feb 2009 16:01:07 -0800 | Subject | Re: [PATCH] cgroups: fix possible use after free | From | Paul Menage <> |
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On Tue, Feb 10, 2009 at 4:45 AM, Al Viro <viro@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@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
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