linux-kernel - Re: [PATH 0/4] usbip: make vhci_hcd.* objects independent of vhci

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Date:   Mon, 5 Mar 2018 10:00:16 +0100
From:   Salvador Fandiño <salvador@...del.com>
To:     shuah@...nel.org, Salvador Fandino <salva@...del.com>,
        linux-usb@...r.kernel.org
Cc:     gregkh@...uxfoundation.org, valentina.manea.m@...il.com,
        linux-kernel@...r.kernel.org, Shuah Khan <shuahkh@....samsung.com>
Subject: Re: [PATH 0/4] usbip: make vhci_hcd.* objects independent of
 vhci_hcd.0

On 02/21/2018 01:35 AM, Shuah Khan wrote:
> Hi Salvador,
> 
> On 01/30/2018 01:36 AM, Salvador Fandino wrote:
>> Let me start by explaining the problem that have motivated me to write
>> this patches:
>>
>> I work on the QVD, a virtual desktop platform for Linux. This software
>> runs Linux desktops (i.e. XFCE, KDE) and their applications inside LXC
>> containers, and makes then available through the network to remote
>> users.
>>
>> Supporting USB devices is a common feature customers have been
>> requesting us for a long time (in order to use, for instance, remote
>> signature pads, bar-code scanners, fingerprint readers, etc.). So, we
>> have been working on that feature using the USB/IP layer on the
>> kernel.
>>
>> Connecting and disconnecting devices and transferring data works
>> seamless for the devices listed above. But we also want to make the
>> usbip operations private to the container where they are run.  For
>> instance, it is unacceptable for our product, that one user could list
>> the devices connected by other users or access them.
>>
>> We can control how can access every device using cgroups once those
>> are attached, but the usbip layer is not providing any mechanism for
>> controlling who can attach, detach or list the devices.
> 
> Did you explore a solution to add a mechanism for access control to
> usbip?

Could you elaborate on that?

For "usbip", do you mean the user space tools? If that is the case, I 
don't think it would be enough. My aim is to limit vhci usage from 
containers and I have no control about what runs inside the containers. 
So, a mangled usbip tool-set could be used by a malicious user to 
circumvent any access control set there.

IMO, there is no other choice but to control access to VHCI at the 
kernel level.


> 
>>
>> So, we got the idea that in order to enforce that remote usbip devices
>> are only visible inside the container where they were imported, we
>> could conveniently mount-bind inside every container just one of the
>> vhci_hcd directories below /sys/devices/platform. So that it is as if
>> every container had a vhci_hcd just for itself (and also, we restrict
>> access to the matching USB ports in cgroups).
>>
>> Unfortunately, all the vhci_hcd.* devices are controlled through
>> attributes in vhci_hcd.0 making our approach fail and so... well, that
>> is what this patch series changes. It makes every vhci_hcd device
>> controllable through attributes inside its own sysfs directory.>
>> The first patch, does that in the kernel, and the second and third
>> patches change user space, adapting the libusbip and the usbip tools
>> code respectively.
>>
>> Then there is a fourth patch, that allows to create much more USB
>> hubs per machine. It was limited to 64 and we are running thousands of
>> containers (every one requiring a hub) per host.
>>
>> These changes are not completely backward compatible. In the sysfs
>> side, besides moving around the attribute files, now the port numbers
>> go from 0 to CONFIG_USBIP_VHCI_HC_PORTS * 2 - 1 and are reused for
>> every vhci_hcd device. I could have maintained the absolute numeration
>> but I think reusing the numbers is a better and simpler approach.
> 
> Not being able to maintain backwards compatibility is an issue. This is
> a considerable change to the user interface.

Well, it is true that it is a considerable change to the user interface 
breaking backward compatibility, but as I had already stated, that 
interface was broken until a couple of months ago, when my coworker, 
Juan Zea, reported it and nobody had noticed it before. So, I don't 
think we are going to affect too many people.

Note also that the user interface does not change when only vhci_hcd.0 
is used.


Regards