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

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Message-ID: <20231002064646.GA1799@lst.de>
Date: Mon, 2 Oct 2023 08:46:46 +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 Tue, Sep 26, 2023 at 10:25:15PM +0100, Al Viro wrote:
> Before your patch: foo_kill_super() calls kill_anon_super(),
> which calls kill_super_notify(), which removes the sucker from
> the list, then frees ->s_fs_info.  After your patch:
> removal from the lists happens via the call of kill_super_notify()
> *after* both of your methods had been called, while freeing
> ->s_fs_info happens from the method call.  IOW, you've restored
> the situation prior to "super: ensure valid info".  The whole
> point of that commit had been to make sure that we have nothing
> in the lists with ->s_fs_info pointing to a freed object.
> 
> It's not about free_anon_bdev(); that part is fine - it's the
> "we can drop the weird second call site of kill_super_notify()"
> thing that is broken.

The point has been to only release the anon dev_t after
kill_super_notify, to prevent two of them beeing reused.

Which we do as the free_anon_bdev is done directly in
deactivate_locked_super.  The new ->free_sb for non-block file systems
frees resources, but none of them matter for sget.