Bug 752375 (CVE-2011-4127)

rhel-5 doesn't implement SG_IO passthrough in virtio-blk (unlike rhel-6) which leaves us with potential local privilege escalation "only".

Statement:

This issue affects the Linux kernel as shipped with Red Hat Enterprise Linux 4,
5, 6, and Red Hat Enterprise MRG. This has been addressed in Red Hat Enterprise Linux 5, 6, and Red Hat Enterprise MRG via https://rhn.redhat.com/errata/RHSA-2012-0107.html, https://rhn.redhat.com/errata/RHSA-2011-1849.html, and https://rhn.redhat.com/errata/RHSA-2012-0333.html. Red Hat Enterprise Linux 4 is now in Production 3 of the maintenance life-cycle, https://access.redhat.com/support/policy/updates/errata/, therefore the fix for this issue is not currently planned to be included in the future updates.

For a storage stack to be vulnerable, every layer must forward the SG_IO
ioctls to the layer below it.

Three device-mapper target types are known to do this: linear, multipath and flakey.

If the I/O has to pass through striped, crypt, mirror or snapshot* device-mapper target types or an md software raid layer on its route to disk, any SG_IO will get blocked by those layers and so we believe such configurations are not vulnerable.

Issue

It was found that using the SG_IO IOCTL to issue SCSI requests to partitions or LVM volumes resulted in the requests being passed to the underlying block device. If a privileged user only had access to a single partition or LVM volume, they could use this flaw to bypass those restrictions and gain read and write access (and be able to issue other SCSI commands) to the entire block device.

In KVM virtualization environments that use raw format virtio disks backed by a
partition or LVM volume, a privileged guest user could use this flaw to
bypass intended access restrictions and issue read and write requests (and other SCSI commands) on the host, and possibly access the data of other guests that reside on the same underlying block device. Partition-based and LVM-based storage pools are not used by default. Refer to the "11. Storage pools" chapter of the Red Hat Enterprise Linux 6 "Virtualization Administration Guide", available from docs.redhat.com, for information about storage pools.

Any driver that uses the SCSI framework is vulnerable. A physical SCSI
device is not required to exploit this issue.

Environment

* Red Hat Enterprise Linux 5 and 6.

* Red Hat Enterprise Virtualization Hypervisor 6.2.

The virtualization aspect of this issue (privileged guests users reading
and writing data on the host and other guests on the same underlying block device) does not affect users of KVM (or the Xen hypervisor) on Red Hat Enterprise Linux 5.

Resolution

The RHSA-2011:1849 update for Red Hat Enterprise Linux 6 corrected this issue.
Future updates will correct this issue for Red Hat Enterprise Linux 5 and Red Hat Enterprise Virtualization Hypervisor 6.2.

It is not possible to use control groups (cgroups) or sVirt mandatory
access control to mitigate this issue. For virtualization hosts (including
Red Hat Enterprise Virtualization Hypervisor 6.2), before updates are applied, the following SystemTap script prevents privileged guest users from issuing
read and write requests (and other SCSI commands) on the host and
possibly access the data of other guests that reside on the same underlying block device.

1) On the host, save the following in a file with the ".stp" extension:

#!/usr/sbin/stap -g
# Requires guru mode (-g) for target variable writes
probe syscall.ioctl {
    SG_IO = 0x2285
    if($cmd == SG_IO && execname() == "qemu-kvm") {
        printf("filtering SG_IO ioctl request from qemu-kvm process %d\n",
pid())
    $cmd   = 0x0
    $fd    = -1
    }
}

2) Install the "systemtap" package and any required dependencies. Refer to
the "2. Using SystemTap" chapter in the Red Hat Enterprise Linux 5 and 6
"SystemTap Beginners Guide" documents, available from docs.redhat.com, for
information on installing the required -debuginfo packages.

3) Run the "stap -g [filename-from-step-1].stp" command as root.

If the host is rebooted, the changes will be lost and the script must be
run again.


Further assistance
If you require assistance with mitigating this issue, contact Red Hat Support:
https://access.redhat.com/support/contact/technicalSupport.html

Created attachment 549226 [details]
CVE-2011-4127-rhel-6-patchset

Created kernel tracking bugs for this issue

Affects: fedora-all [bug 769911]

This issue has been addressed in following products:

  RHEV-H and Agents for RHEL-6

Via RHSA-2011:1850 https://rhn.redhat.com/errata/RHSA-2011-1850.html

This issue has been addressed in following products:

  Red Hat Enterprise Linux 6

Via RHSA-2011:1849 https://rhn.redhat.com/errata/RHSA-2011-1849.html

Upstream discussion:

https://lkml.org/lkml/2011/12/22/270

(Comment 53 content copied to knowledgebase: https://access.redhat.com/kb/docs/DOC-67874)

This issue has been addressed in following products:

  Red Hat Enterprise Linux 5

Via RHSA-2012:0107 https://rhn.redhat.com/errata/RHSA-2012-0107.html

This issue has been addressed in following products:

  MRG for RHEL-6 v.2

Via RHSA-2012:0333 https://rhn.redhat.com/errata/RHSA-2012-0333.html

This issue has been addressed in following products:

  Red Hat Enterprise Linux 5.6 EUS - Server Only

Via RHSA-2012:0358 https://rhn.redhat.com/errata/RHSA-2012-0358.html