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    SubjectRe: [PATCH 2/3] block: fail SCSI passthrough ioctls on partition devices
    On Thu, Dec 22, 2011 at 3:48 PM, Alasdair G Kergon <agk@redhat.com> wrote:
    > On Thu, Dec 22, 2011 at 02:25:56PM -0800, Linus Torvalds wrote:
    >> I don't *think* anybody does something as crazy as giving actual block
    >> device ownership to people,
    >
    > That can happen when running virtual machines backed by logical volumes.

    So my worry is not so much the security fix for the vm case, but
    simply normal people - who don't hit the issue to begin with - now
    being screwed because what their distro does no longer works.

    For example, I just traced it, and "eject /dev/sdb1" does a CDROMEJECT
    ioctl when used as the root user. I haven't tested the patch, but just
    reading it, I'd expect it to break that.

    And that's the *natural* way to eject a mounted device. Look at the
    USB memory sticks you have. They are almost all partitioned to have
    one partition, and that one partition doesn't cover the whole device.
    And it's that one partition you use to interact with it - it's what
    you mount, and what you eject.

    Breaking things like that is not an option. It's stuff people do every
    day. And there may be some very non-obvious reason that I don't see
    why it's not broken by this patch-series, but that's the kind of thing
    that I worry about.

    And I worry about it a *lot* more than I worry about some broken
    virtualization setup where you have system engineers that could patch
    their own kernel if they feel strongly about this. We simply
    *must*not* break things.

    I absolutely do not get the feeling that this has been tested so much
    and is so obvious that there is no risk of breakage.

    I suspect one thing that would be reasonable would be to just say
    "Root can do any ioctl's he damn well wants on a partial device". That
    would make me worry much less about the "normal" setup breaking where
    you don't give regular users direct access to partitions to begin
    with.

    But even that might not work reliably. Maybe the system deamon that
    actually does the eject (when a normal user does an eject) has been
    setguid to 'disk' instead, and isn't root. I don't know. I doubt you
    know either. Maybe some of them have fallback code to find the parent
    device. Do you know? Do you know that all do? I doubt it.

    Linus


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