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

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Message-ID: <20230926093834.GB13806@lst.de>
Date: Tue, 26 Sep 2023 11:38:34 +0200
From: Christoph Hellwig <hch@....de>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Christoph Hellwig <hch@....de>, Christian Brauner <brauner@...nel.org>,
	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 Thu, Sep 14, 2023 at 12:27:12AM +0100, Al Viro wrote:
> On Wed, Sep 13, 2023 at 08:09:57AM -0300, Christoph Hellwig wrote:
> > Releasing an anon dev_t is a very common thing when freeing a
> > super_block, as that's done for basically any not block based file
> > system (modulo the odd mtd special case).  So instead of requiring
> > a special ->kill_sb helper and a lot of boilerplate in more complicated
> > file systems, just release the anon dev_t in deactivate_locked_super if
> > the super_block was using one.
> > 
> > As the freeing is done after the main call to kill_super_notify, this
> > removes the need for having two slightly different call sites for it.
> 
> Huh?  At this stage in your series freeing is still in ->kill_sb()
> instances, after the calls of kill_anon_super() you've turned into
> the calls of generic_shutdown_super().

The above refers to freeing the anon dev_t, which at this stage is done
right after the kill_super_notify in generic_shutdown_super.

> You do split it off into a separate method later in the series, but
> at this point you are reopening the same UAF that had been dealt with
> in dc3216b14160 "super: ensure valid info".

How?

Old sequence before his patch:

	deactivate_locked_super()
	  -> kill_anon_super()
	    -> generic_shutdown_super()
	    -> kill_super_notify()
	    -> free_anon_bdev()
	  -> kill_super_notify()

New sequence with this patch:

	deactivate_locked_super()
	  -> generic_shutdown_super()
	    -> kill_super_notify()
	    -> free_anon_bdev()