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    SubjectRe: [PATCH 2/3] block: fail SCSI passthrough ioctls on partition devices
    On 12/22/2011 07:37 PM, Linus Torvalds wrote:
    > On Thu, Dec 22, 2011 at 10:02 AM, Paolo Bonzini<pbonzini@redhat.com> wrote:
    >> Linux allows executing the SG_IO ioctl on a partition or even on an
    >> LVM volume, and will pass the command to the underlying block device.
    >> This is well-known, but it is also a large security problem when (via
    >> Unix permissions, ACLs, SELinux or a combination thereof) a program or
    >> user needs to be granted access to a particular partition or logical
    >> volume but not to the full device.
    >
    > So who actually *does* this in practice?

    Virtualization, as explained in the cover letter.

    >> + /* In particular, rule out all resets and host-specific ioctls. */
    >> + return -ENOTTY;
    >
    > This kind of crazy needs to go away.

    What crazy? It's not a permission problem. Sending a SCSI command to a
    partition makes no sense. A permission problem implies that somehow you
    should be able to fix it by granting additional permissions, which is
    not the case here.

    > If it's a permission problem, state that. Don't turn it into ENOTTY that then:
    >
    >> + return ret == -ENOTTY ? -ENOIOCTLCMD : ret;
    >
    > gets turned into another random error number.

    That's existing craziness of the compat_ioctl mechanism:

    /* Most of the generic ioctls are handled in the normal fallback path.
    This assumes the blkdev's low level compat_ioctl always returns
    ENOIOCTLCMD for unknown ioctls. */

    The logic is quite intricate:

    1. process generic block layer ioctls that require compat handling
    (compat_blkdev_ioctl)

    2. process device-specific ioctls that require special 32-on-64
    handling, whose implementation is outside block/ (sd_compat_ioctl).

    3. process device-specific ioctls that require special 32-on-64
    handling, whose implementation is in block/compat_ioctl.c
    (compat_blkdev_driver_ioctl).

    4. fallback to the normal ioctl implementation for ioctls that do not
    require 32-on-64 (__blkdev_driver_ioctl).

    If I return ENOTTY (or EPERM for that matter: anything but ENOIOCTLCMD),
    then I rule out execution of steps 3 and especially 4. This means
    32-on-64 systems will get ENOTTY for BLKGETSIZE64 and will fail to boot.

    Paolo


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