linux-kernel - Re: [PATCH v2] writeback, cgroup: fix null-ptr-deref write in bdi_split_work_to

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Date:   Mon, 10 Apr 2023 20:53:17 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Baokun Li <libaokun1@...wei.com>
Cc:     <linux-fsdevel@...r.kernel.org>, <viro@...iv.linux.org.uk>,
        <brauner@...nel.org>, <jack@...e.cz>, <tj@...nel.org>,
        <dennis@...nel.org>, <adilger.kernel@...ger.ca>,
        <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com>,
        <yangerkun@...wei.com>, <houtao1@...wei.com>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH v2] writeback, cgroup: fix null-ptr-deref write in
 bdi_split_work_to_wbs

On Mon, 10 Apr 2023 21:08:26 +0800 Baokun Li <libaokun1@...wei.com> wrote:

> 
> ...
>
> To solve this problem, percpu_ref_exit() is called under RCU protection
> to avoid race between cgwb_release_workfn() and bdi_split_work_to_wbs().
> Moreover, replace wb_get() with wb_tryget() in bdi_split_work_to_wbs(),
> and skip the current wb if wb_tryget() fails because the wb has already
> been shutdown.
> 
> Fixes: b817525a4a80 ("writeback: bdi_writeback iteration must not skip dying ones")
> Fixes: f3b6a6df38aa ("writeback, cgroup: keep list of inodes attached to bdi_writeback")

Two Fixes: is awkward.  The Fixes: serves a guide to which kernel
versions should be patched, but those two commits are six years apart.

So... how far back should this fix be backported?

>  fs/fs-writeback.c | 17 ++++++++++-------
>  mm/backing-dev.c  | 12 ++++++++++--

Jens, which tree do you think should carry this?