Re: [PATCH] USB: hub: make sure stale buffers are not enumerated

[Johan Hovold <johan@kernel.org>]

On Mon, Jul 24, 2023 at 02:40:57PM +0200, Oliver Neukum wrote:
> Quoting Alan Stern on why we cannot just check errors:
> 
> The operation carried out here is deliberately unsafe (for full-speed
> devices).  It is made before we know the actual maxpacket size for ep0,
> and as a result it might return an error code even when it works okay.
> This shouldn't happen, but a lot of USB hardware is unreliable.
> 
> Therefore we must not ignore the result merely because r < 0.  If we do
> that, the kernel might stop working with some devices.
> 
> He is absolutely right. However, we must make sure that in case
> we read nothing or a short answer, the buffer contains nothing
> that can be misinterpreted as a valid answer.
> So we have to zero it before we use it for IO.

This patch is neither correct or needed. The current implementation sets
	
	buf->bMaxPacketSize0 = 0

before reading the descriptor and makes sure that that field is non-zero
before accessing buf->bDescriptorType which lies before bMaxPacketSize0.

It may be subtle, but it looks correct.

Johan