Messages in this thread | | | Subject | Re: [PATCH v1 0/1] ioctl to disallow detaching kernel USB drivers | From | Krzysztof Opasiak <> | Date | Mon, 30 Nov 2015 19:48:56 +0100 |
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On 11/30/2015 06:20 PM, Greg KH wrote: > On Mon, Nov 30, 2015 at 06:12:22PM +0100, Krzysztof Opasiak wrote: >> >> >> On 11/30/2015 05:16 PM, Alan Stern wrote: >>> On Fri, 27 Nov 2015, Krzysztof Opasiak wrote: >>> >>>>>> I run through your code and as far as I understand above is not exactly >>>>>> true. Your patch allows only to prevent userspace from accessing interfaces >>>>>> which has kernel drivers, there is no way to stop an application from taking >>>>>> control over all free interfaces. >>>>>> >>>>>> Let's say that your device has 3 interfaces. First of them has a kernel >>>>>> driver but second and third doesn't. You have 2 apps. One should communicate >>>>>> using second interface and another one third. But first app is malicious and >>>>>> it claims all free interfaces of received device (your patch doesn't prevent >>>>>> this). And when second app starts it is unable to do anything with the >>>>>> device because all interfaces are taken. How would you like to handle this? >>>>> >>>>> You can't, and why would you ever want to, as you can't tell what an app >>>>> "should" or "should not" do. If you really care about this, then use a >>>>> LSM policy to prevent this. >>>> >>>> Well, an app can declare what it does and what it needs in it's manifest >>>> file (or some equivalent of this) and the platform should ensure that >>>> app can do only what it has declared. >>>> >>>> I would really like to use LSM policy in here but currently it is >>>> impossible as one device node represents whole device. Permissions (even >>>> those from LSM) are being checked only on open() not on each ioctl() so >>>> as far as I know there is nothing which prevents any owner of opened fd >>>> to claim all available (not taken by someone else) interfaces and LSM >>>> policy is unable to filter those calls (unless we add some LSM hooks >>>> over there). >>> >>> How about this approach? Once a process has dropped its usbfs >>> privileges, it's not allowed to claim any interfaces (either explicitly >>> or implicitly). Instead, it or some manager program must claim the >>> appropriate interfaces before dropping privileges. >>> >> >> I agree that restricting interface claiming only to privileged process is a >> good idea. Unfortunately this generates a problem when program needs more >> than one interface (like in cdc - data + control for example). We need to >> declare both of them in first call to "usb-manager" or reopen the dev node >> at second call and claim all interfaces claimed using this fd till now and >> claim one more and then drop privileges and send a new fd. > > Have you seen such a device that is controlled this way in userspace? > Don't over-engineer something that is probably pretty rare... >
Yes I have seen such devices (not cdc of course) and they were driven using libusb (vendor specific service + "driver" to bypass publishing protocol code due to kernel's GPL). I have even seen an android app written in java which claims and uses multiple interfaces using android's USB API, so it's real;)
-- Krzysztof Opasiak Samsung R&D Institute Poland Samsung Electronics
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