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

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Message-ID: <20230914-zielt-einzog-00389009b293@brauner>
Date:   Thu, 14 Sep 2023 09:56:57 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     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

> BTW, this part of commit message in 2c18a63b760a is rather confused:
>     Recent rework moved block device closing out of sb->put_super() and into
>     sb->kill_sb() to avoid deadlocks as s_umount is held in put_super() and
>     blkdev_put() can end up taking s_umount again.
> 
> That was *NOT* what a recent rework had done.  Block device closing had never
> been inside ->put_super() - at no point since that (closing, that is) had been
> introduced back in 0.97 ;-)  ->put_super() predates it (0.95c+).

I think the commit message probably just isn't clear enough. The main
block device of a superblock isn't closed in sb->put_super(). That's
always been closed in kill_block_super() after generic_shutdown_super().

But afaict filesystem like ext4 and xfs may have additional block
devices open exclusively and closed them in sb->put_super():

xfs_fs_put_super()
-> xfs_close_devices()
   -> xfs_blkdev_put()
      -> blkdev_put()

ext4_put_super()
-> ext4_blkdev_remove()
   -> blkdev_put()