Re: [PATCH v2 0/7] bdi: fix use-after-free for bdi device

["Theodore Y. Ts'o" <tytso@mit.edu>]

On Wed, Mar 04, 2020 at 06:29:07PM +0100, Greg KH wrote:
> The rule should be, "whenever you pass a pointer to a device off, the
> reference count is incremented".  Somehow that is not happening here and
> RCU is not going to solve the issue really, it's only going to delay the
> problem from showing up until much later.
> ...
> The struct device refcount should be all that is needed, don't use RCU
> just to "delay freeing this object until some later time because someone
> else might have a pointer to id".  That's ripe for disaster.

I agree that this is a better fix than trying to continue to paper
over the problem.

That being said, I also think it would be better if we could *also*
send a notification to the file system (or device mapper) when a block
device has disappeared, so we can set a flag in struct super
indicating, "this is an ex-device" so that we don't have to have
potentially hundreds of I/O errors clogging up the console and/or any
error notification ifrastructure we might want to add in the future,
as we attempt to send I/O to a device is not coming back.  This would
allow us to short-circuit things like writeback, instead of letting
everything drain via pointless io_submits sending bios that will never
go anywhere useful.

					- Ted