Re: [PATCH 03/19] fs: release anon dev_t in deactivate_locked_super

[Christoph Hellwig]

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()