Date:	Mon, 08 Feb 2016 21:00:05 +0100
From:	Nicolai Stange <nicstange@...il.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Nicolai Stange <nicstange@...il.com>,
	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

Greg Kroah-Hartman <gregkh@...uxfoundation.org> writes:

> On Mon, Feb 08, 2016 at 06:14:58PM +0100, Nicolai Stange wrote:
>> Greg Kroah-Hartman <gregkh@...uxfoundation.org> writes:
>> 
>> > 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?
>> 
>> You mean in the form of a patch [3/3] for an arbitrary subsystem other
>> than debugfs? Or in the form of an update of
>> Documentation/filesystems/debugfs.txt?
>
> For an arbritary subsystem would be great.  Showing how this should be
> used / converted tree-wide.
>
>> In case you want to have a patch: for the DEFINE_DEBUGFS_ATTRIBUTE, I
>> could simply abuse
>>   drivers/staging/android/ion/ion.c
>> as it has got a DEFINE_SIMPLE_ATTRIBUTE debug_shrink_fops passed to
>> debugfs. In this particular case, it even looks like that this debugfs
>> file can be removed through ion_client_destroy() without any module
>> removal. Fixing this would be as easy as
>> s/DEFINE_SIMPLE_ATTRIBUTE/DEFINE_DEBUGFS_ATTRIBUTE/.
>
> Great, why wouldn't we do that for all users of debugfs that have this
> type of interaction with it?

So this is a "yes", I should include these kind of fixes within this
series as [3/X], [4/X], ..., [X/X]?

Last time I checked the tree (Nov.), there weren't any users of this
kind (debugfs file removal w/o module unload).
Obviously I missed ion though... I will recheck.

>
>> Regarding a use case with custom made file_operations whose
>> reader and writer are protected by the debugfs_file_use_data_*()
>> helpers, I'm a little bit at a loss with: ion.c has got its custom
>> 'debug_heap_fops', but in this case, it would probably be more
>> appropriate to create a general debugfs_create_seqfile() centrally in
>> debugfs.
>
> ion is 'rough', but if enough people use seqfile in debugfs, yes, we
> should provide a generic interface for it to make it easier to use so
> they don't have to roll their own, and so they get the fixes you did
> here for their code as well.

A quick check revealed that there are *many* such seqfile users.

Since these would all get touched, I think it is better to postpone the
introduction of a debugfs_create_seqfile() to another series dedicated
to that?

Thank you,

Nicolai

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