Re: [Spice-devel] [RFC PATCH 1/1] Add a usbredir kernel module to remotely connect USB devices over IP.

Oliver Neukum
2015-07-02T18:48:15Z

On Thu, 2015-07-02 at 10:57 -0500, Jeremy White wrote:
> On 07/02/2015 07:10 AM, Oliver Neukum wrote:
> > On Thu, 2015-07-02 at 13:35 +0200, Hans de Goede wrote:
> >> Hi,
> >>
> >> On 02-07-15 10:45, Oliver Neukum wrote:
> >>> On Wed, 2015-07-01 at 10:06 +0100, Daniel P. Berrange wrote:
> >>>
> >>>> I don't really think it is sensible to be defining & implementing new
> >>>> network services which can't support strong encryption and authentication.
> >>>> Rather than passing the file descriptor to the kernel and having it do
> >>>> the I/O directly, I think it would be better to dissassociate the kernel
> >>>> from the network transport, and thus leave all sockets layer data I/O
> >>>> to userspace daemons so they can layer in TLS or SASL or whatever else
> >>>> is appropriate for the security need.
> >>>
> >>> Hi,
> >>>
> >>> this hits a fundamental limit. Block IO must be done entirely in kernel
> >>> space or the system will deadlock. The USB stack is part of the block
> >>> layer and the SCSI error handling. Thus if you involve user space you
> >>> cannot honor memory allocation with GFP_NOFS and you break all APIs
> >>> where we pass GFP_NOIO in the USB stack.
> >>>
> >>> Supposed you need to reset a storage device for error handling.
> >>> Your user space programm does a syscall, which allocates memory
> >>> and needs to launder pages. It proceeds to write to the storage device
> >>> you wish to reset.
> >>>
> >>> It is the same problem FUSE has with writable mmap. You cannot do
> >>> block devices in user space sanely.
> >>
> >> So how is this dealt with for usbip ?
> >
> > As far as I can tell, it isn't. Running a storage device over usbip
> > is a bit dangerous.
>
> I don't follow that analysis. The usbip interactions with the usb stack
> all seem to be atomic, and never trigger a syscall, as far as I can
> tell. A port reset will flip a few bits and return. A urb enqueue
> queues and wakes a different thread, and returns. The alternate thread
> performs the sendmsg.
>
> I'm not suggesting that running a storage device over usbip is
> especially safe, but I don't see the limit on the design.

Are you referring to the current code or the proposed user space pipe?

Regards
Oliver