linux-kernel - Re: [PATCH v2 2/2] debugfs: prevent access to removed files' private data

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Date:	Mon, 8 Feb 2016 08:17:29 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Nicolai Stange <nicstange@...il.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Jonathan Corbet <corbet@....net>, Jan Kara <jack@...e.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] debugfs: prevent access to removed files' private
 data

On Mon, Feb 08, 2016 at 04:03:27PM +0100, Nicolai Stange wrote:
> Upon return of debugfs_remove()/debugfs_remove_recursive(), it might
> still be attempted to access associated private file data through
> previously opened struct file objects. If that data has been freed by
> the caller of debugfs_remove*() in the meanwhile, the reading/writing
> process would either encounter a fault or, if the memory address in
> question has been reassigned again, unrelated data structures could get
> overwritten.
> 
> However, since debugfs files are seldomly removed, usually from module
> exit handlers only, the impact is very low.
> 
> Since debugfs_remove() and debugfs_remove_recursive() are already
> waiting for a SRCU grace period before returning to their callers,
> enclosing the access to private file data from ->read() and ->write()
> within a SRCU read-side critical section does the trick:
> - Introduce the debugfs_file_use_data_start() and
>   debugfs_file_use_data_finish() helpers which just enter and leave
>   a SRCU read-side critical section. The former also reports whether the
>   file is still alive, that is if d_delete() has _not_ been called on
>   the corresponding dentry.
> - Introduce the DEFINE_DEBUGFS_ATTRIBUTE() macro which is completely
>   equivalent to the DEFINE_SIMPLE_ATTRIBUTE() macro except that
>   ->read() and ->write are set to SRCU protecting wrappers around the
>   original simple_read() and simple_write() helpers.
> - Use that DEFINE_DEBUGFS_ATTRIBUTE() macro for all debugfs_create_*()
>   attribute creation variants where appropriate.
> - Manually introduce SRCU protection to the debugfs-predefined readers
>   and writers not covered by the above DEFINE_SIMPLE_ATTRIBUTE()->
>   DEFINE_DEBUGFS_ATTRIBUTE() replacement.
> 
> Finally, it should be worth to note that in the vast majority of cases
> where debugfs users are handing in a "custom" struct file_operations
> object to debugfs_create_file(), an attribute's associated data's
> lifetime is bound to the one of the containing module and thus,
> taking a reference on ->owner during file opening acts as a proxy here.
> There is no need to do a mass replace of DEFINE_SIMPLE_ATTRIBUTE() to
> DEFINE_DEBUGFS_ATTRIBUTE() outside of debugfs.
> 
> OTOH, new users of debugfs are encouraged to prefer the
> DEFINE_DEBUGFS_ATTRIBUTE() macro over DEFINE_SIMPLE_ATTRIBUTE() and it,
> as well as the needed read/write wrappers are made available globally.
> For new users implementing their own readers and writers, the lifetime
> management helpers debugfs_file_use_data_start() and
> debugfs_file_use_data_finish() are exported.

Nice job.  One more request... :)

Can you show how you would convert a subsystem to use these new
macros/calls to give a solid example of it in use outside of the debugfs
core?

thanks,

greg k-h