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

[Jeremy White <jwhite@codeweavers.com>]

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.

Cheers,

Jeremy