Re: [PATCH bpf] bpf: cgroup: prevent out-of-order release of cgroup bpf

Roman Gushchin
2020-01-04T01:13:42Z

On Fri, Jan 03, 2020 at 04:35:25PM -0800, Alexei Starovoitov wrote:
> On Fri, Dec 27, 2019 at 01:50:34PM -0800, Roman Gushchin wrote:
> > Before commit 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf
> > from cgroup itself") cgroup bpf structures were released with
> > corresponding cgroup structures. It guaranteed the hierarchical order
> > of destruction: children were always first. It preserved attached
> > programs from being released before their propagated copies.
> >
> > But with cgroup auto-detachment there are no such guarantees anymore:
> > cgroup bpf is released as soon as the cgroup is offline and there are
> > no live associated sockets. It means that an attached program can be
> > detached and released, while its propagated copy is still living
> > in the cgroup subtree. This will obviously lead to an use-after-free
> > bug.
> ...
> > @@ -65,6 +65,9 @@ static void cgroup_bpf_release(struct work_struct *work)
> >
> > mutex_unlock(&cgroup_mutex);
> >
> > + for (p = cgroup_parent(cgrp); p; p = cgroup_parent(p))
> > + cgroup_bpf_put(p);
> > +
>
> The fix makes sense, but is it really safe to walk cgroup hierarchy
> without holding cgroup_mutex?

It is, because we're holding a reference to the original cgroup and going
towards the root. On each level the cgroup is protected by a reference
from their child cgroup.

Thanks!