linux-kernel - Re: [PATCH] cgroup: handle cset multiidentity issue when migration

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Message-ID: <20220608135110.GA19399@blackbody.suse.cz>
Date:   Wed, 8 Jun 2022 15:51:10 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     shisiyuan <shisiyuan19870131@...il.com>
Cc:     tj@...nel.org, hannes@...xchg.org, lizefan@...wei.com,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        shisiyuan <shisiyuan@...omi.com>
Subject: Re: [PATCH] cgroup: handle cset multiidentity issue when migration

Hello.

On Fri, Jun 03, 2022 at 12:34:48AM +0800, shisiyuan <shisiyuan19870131@...il.com> wrote:
> Bug code flow:
> cset X's initial refcount is 1.
> 1. cgroup_attach_task()
> 2. [For thread1]
>    cgroup_migrate_add_src()
>    [For thread2]
>    cgroup_migrate_add_src()
>      cset X is thread2's src_cset , ref->2,
>      and its mg_preload_node is added to
>      mgctx->preloaded_src_csets.
> 3. cgroup_migrate_prepare_dst()
>    [For thread1]
>    find_css_set()
>      cset X is thread1's dst_cset, ref->3
>    put_css_set()
>      ref->2 because cset X's mg_preload_node is not
>      empty(already in mgctx->preloaded_src_csets).
>    [For thread2]
>    find_css_cset()
>      cset X is also thread2's dst_cset, ref->3
>      then drop src_cset, ref->1
> [cgroup_free] ref->0
> 4. cgroup_migrate_execute
>    [For thread1]
>    ref -> 0xc0000000(UAF)

I'm trying to understand when this happens.
You migrate a process with two threads while one of them exits?

This should be properly synchronized with cgroup_threadgroup_rwsem, so I
don't understand where does the [cgroup_free] between 3. and 4. come
from.

Do you have a reproducer for this?

Thanks,
Michal