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

[Chen Ridong <chenridong@huaweicloud.com>]

On 2024/10/17 1:04, Tejun Heo wrote:
> 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.
> 

So, should commit 04f8ef5643bc ("cgroup: Fix memory leak caused by 
missing cgroup_bpf_offline") be reverted, and should cgroup_bpf_inherit 
be only called in v2?
Have I understood this correctly?

Best regards,
Ridong