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

[Alexei Starovoitov]

On Sat, Jan 04, 2020 at 01:13:24AM +0000, Roman Gushchin wrote:
> 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.

cgroup_bpf_put(p) can make bpf.refcnt zero which may call cgroup_bpf_release()
on another cpu which will do cgroup_put() and this cpu p = cgroup_parent(p)
would be use-after-free?
May be not due to the way work_queues are implemented.
But it feels dangerous to have such delicate release logic.
Why not to move the loop under the mutex and make things obvious?