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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Feb 08, 2016 at 09:00:05PM +0100, Nicolai Stange wrote:
> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
> 
> > On Mon, Feb 08, 2016 at 06:14:58PM +0100, Nicolai Stange wrote:
> >> Greg Kroah-Hartman <gregkh@linuxfoundation.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]?

Yes please.

> 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?

Yes that would be a good idea.

thanks,

greg k-h