linux-hardening - Re: [PATCH 03/19] fs: release anon dev_t in deactivate_locked

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Date:   Fri, 15 Sep 2023 15:28:14 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Tejun Heo <tj@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Damien Le Moal <dlemoal@...nel.org>,
        Naohiro Aota <naohiro.aota@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-hardening@...r.kernel.org,
        cgroups@...r.kernel.org
Subject: Re: [PATCH 03/19] fs: release anon dev_t in deactivate_locked_super

On Fri, Sep 15, 2023 at 04:12:07PM +0200, Christian Brauner wrote:
> +	static void some_fs_kill_sb(struct super_block *sb)
> +	{
> +		struct some_fs_info *info = sb->s_fs_info;
> +
> +		kill_*_super(sb);
> +		kfree(info);
> +	}
> +
> +It's best practice to never deviate from this pattern.

The last part is flat-out incorrect.  If e.g. fatfs or cifs ever switches
to that pattern, you'll get UAF - they need freeing of ->s_fs_info
of anything that ever had been mounted done with RCU delay; moreover,
unload_nls() in fatfs needs to be behind the same.

Lifetime rules for fs-private parts of superblock are really private to
filesystem; their use by sget/sget_fc callbacks might impose restrictions
on those, but that again is none of the VFS business.