linux-kernel - Re: [PATCH] cgroup/bpf: fix NULL pointer dereference at cgroup_bpf

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Message-ID: <bidpqhgxflkaj6wzhkqj5fqoc2zumf3vcyidspz4mqm4erq3bu@r4mzs45sbe7g>
Date: Wed, 16 Oct 2024 15:13:52 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Chen Ridong <chenridong@...weicloud.com>
Cc: tj@...nel.org, lizefan.x@...edance.com, hannes@...xchg.org, 
	longman@...hat.com, john.fastabend@...il.com, roman.gushchin@...ux.dev, 
	quanyang.wang@...driver.com, ast@...nel.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, chenridong@...wei.com, 
	wangweiyang2@...wei.com
Subject: Re: [PATCH] cgroup/bpf: fix NULL pointer dereference at
 cgroup_bpf_offline

Hello.

On Wed, Oct 16, 2024 at 09:36:33AM GMT, Chen Ridong <chenridong@...weicloud.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@...wei.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?

Thanks,
Michal

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