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    SubjectRe: setting up CDB filters in udev (was Re: [PATCH v2 0/3] block: add queue-private command filter, editable via sysfs)
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    Paolo

    Il 25/10/2012 20:35, Paolo Bonzini ha scritto:
    >> On Thu, Oct 25, 2012 at 09:37:39AM +0200, Paolo Bonzini wrote:
    >>> Il 24/10/2012 18:47, Tejun Heo ha scritto:
    >>>> So, I'm still not convinced we need to go forward with full
    >>>> configurability. All use cases you described can be covered with
    >>>> per-class static filters + simple override switch to disable all,
    >>>> which would result in a lot simpler implementation w/ much
    >>>> smaller userland interface.
    >>>
    >>> I'm not sure the userland interface would be smaller, and it would
    >>> be more complex to get right:
    >>>
    >>> 1) how do you override the default? ioctl+SCM_RIGHTS or sysfs?
    >>
    >> Disabling filters if opened by root and tranfering via SCM_RIGHTS
    >> would be the simplest interface-wise (there's no new interface at
    >> all). Would that be too dangerous security-wise?
    >
    > That would be a change with respect to what we have now. After
    > transferring a root-opened (better: CAP_SYS_RAWIO-opened) file
    > descriptor to an unprivileged process your SG_IO commands get
    > filtered. So a ioctl is needed if you want to rely on SCM_RIGHTS.
    >
    >>> 2) do you need to override the default to "no access", "full
    >>> access" and "default access", or is a binary knob (default
    >>> access/full access) sufficient?
    >>
    >> Default / full should be enough, no?
    >
    > If a ioctl has to be added, I'd rather have at least none/full/default.
    >
    >>> 3) what capabilities control the setting?
    >>
    >> CAP_SYS_RAWIO seems to be a pretty good fit.
    >
    > Yes, for a ioctl it is (for sysfs CAP_SYS_ADMIN is better IMHO).
    >
    >> I guess I just feel quite reluctant to expose another rather obscure
    >> userland configurable in-kernel filter and at the same time I'm not
    >> sure whether this is flexible enough. What if a device is shared by
    >> multiple virtual machines which are trusted at different levels?
    >
    > No, you just don't do that. If a device is passed through to virtual
    > machines, it is between similar virtual machines (for some definition
    > of similar). The only case where you have this sharing is in practice
    > if either the device is read-only (my patch does give you a basic
    > two-level filtering, with two separate bitmaps for RO and RW) or if you
    > allow persistent reservations (which is as close to full trust as you
    > can get).
    >
    >> I'm not trying to block it at all cost but let's make sure we looked
    >> into most possibilities before (re)adding this userland visible
    >> interface.
    >
    > Sure, understood. :)
    >
    >> Jens, James, what do you guys think?
    >
    > Paolo
    >



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