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Date:   Sat, 8 Oct 2022 07:46:07 +0200
From:   Christian Brauner <brauner@kernel.org>
To:     Tejun Heo <tj@kernel.org>
Cc:     syzbot <syzbot+534ee3d24c37c411f37f@syzkaller.appspotmail.com>,
        gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] general protection fault in kernfs_get_inode
Message-ID: <20221008054607.fyqzd7oil2ulzf34@wittgenstein>
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On Fri, Oct 07, 2022 at 11:35:49AM -1000, Tejun Heo wrote:
> (cc'ing Christian and quoting whole body)
> 
> Christan, I can't repro it here but think what we need is sth like the
> following. The problem is that cgroup_is_dead() check in the fork path isn't
> enough as the perm check depends on cgrp->procs_file being available but
> that is cleared while DYING before DEAD. So, depending on the timing, we can
> end up trying to deref NULL pointer in may_write.
> 
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 8ad2c267ff471..603b7167450a1 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -4934,6 +4934,10 @@ static int cgroup_may_write(const struct cgroup *cgrp, struct super_block *sb)
>  
>  	lockdep_assert_held(&cgroup_mutex);
>  
> +	/*if @cgrp is being removed, procs_file may already be gone */
> +	if (!cgrp->procs_file.kn)
> +		return -ENODEV;
> +
>  	inode = kernfs_get_inode(sb, cgrp->procs_file.kn);
>  	if (!inode)
>  		return -ENOMEM;

Tejun, thanks for the Cc.
Yep, that seems to be the correct analysis.
I assume you're turning this into a proper patch, so:

Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>