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

[Al Viro]

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.

More detailed example: take a look at NFS.  We have ->get_tree() there
call sget_fc() with nfs_compare_super() as possible 'test' callback.
It does look at ->s_fs_info of the superblocks found on the list
of instances for fs type in question.  Moreover, it proceeds to
call nfs_compare_mount_options(), which chases pointers from that
(at the very least fetch ->client in nfs_server instance ->s_fs_info
points to and dereferences that).

We really, really do not want nfs_free_server() happen while the
superblock is visible in the instances list.  Now, in your tree
nfs_free_sb() call nfs_free_server().  *Without* having called
kill_super_notify() first - you do that only after the call of
->free_sb().

So with this series applied we have UAF on race between mount and
umount.  For NFS.  No block devices involved.

Old logics had been "after generic_shutdown_super() the private
parts of superblock belong to filesystem alone; they might be
accessed by methods called from RCU pathwalk, but that's it".

I still don't see any clear rules for the new one.  And the more
I'm looking, the more sceptical I get about the approach you've
taken, TBH...