Re: [PATCH] cgroup/bpf: fix NULL pointer dereference at cgroup_bpf_offline

[Tejun Heo <tj@kernel.org>]

On Wed, Oct 16, 2024 at 03:13:52PM +0200, Michal Koutný wrote:
> Hello.
> 
> On Wed, Oct 16, 2024 at 09:36:33AM GMT, Chen Ridong <chenridong@huaweicloud.com> wrote:
> > As mentioned above, when cgroup_bpf_inherit returns an error in
> > cgroup_setup_root, cgrp->bpf.refcnt has been exited. If cgrp->bpf.refcnt is
> > killed again in the cgroup_kill_sb function, the data of cgrp->bpf.refcnt
> > may have become NULL, leading to NULL pointer dereference.
> > 
> > To fix this issue, goto err when cgroup_bpf_inherit returns an error.
> > Additionally, if cgroup_bpf_inherit returns an error after rebinding
> > subsystems, the root_cgrp->self.refcnt is exited, which leads to
> > cgroup1_root_to_use return 1 (restart) when subsystems is  mounted next.
> > This is due to a failure trying to get the refcnt(the root is root_cgrp,
> > without rebinding back to cgrp_dfl_root). So move the call to
> > cgroup_bpf_inherit above rebind_subsystems in the cgroup_setup_root.
> > 
> > Fixes: 04f8ef5643bc ("cgroup: Fix memory leak caused by missing cgroup_bpf_offline")
> > Signed-off-by: Chen Ridong <chenridong@huawei.com>
> 
> Hm, I always thought that BPF progs can only be attached to the default
> hierarchy (cgroup_bpf_prog_attach/cgroup_get_from_fd should prevent
> that).
> 
> Thus I wonder whether cgroup_bpf_inherit (which is more like
> cgroup_bpf_init in this case) needs to be called no v1 roots at all (and
> with such a change, 04f8ef5643bc could be effectively reverted too).
> 
> Or can bpf data be used on v1 hierarchies somehow?

We relaxed some of the usages (see cgroup_v1v2_get_from_fd()) but cgroup BPF
progs can only be attached to v2.

Thanks.

-- 
tejun