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

[Alexei Starovoitov]

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?