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---===[ Qubes Security Bulletin #30 ]===--- | |
May 2, 2017 | |
Critical Xen bugs related to PV memory virtualization (XSA-213, XSA-214) | |
Summary | |
======== | |
Today the Xen Security Team has disclosed two bugs related to PV memory | |
handling affecting Qubes OS: XSA-213 [1] and XSA-214 [2]. | |
An attacker who exploits either of these bugs can break Qubes-provided | |
isolation. This means that if an attacker has already exploited another | |
vulnerability, e.g. in a Web browser or networking or USB stack, then | |
the attacker would be able to compromise a whole Qubes system. | |
Technical details | |
================== | |
Xen Security Advisory XSA-213 [1]: | |
| x86: 64bit PV guest breakout via pagetable use-after-mode-change | |
| | |
| 64-bit PV guests typically use separate (root) page tables for their | |
| kernel and user modes. Hypercalls are accessible to guest kernel | |
| context only, which certain hypercall handlers make assumptions on. | |
| The IRET hypercall (replacing the identically name CPU instruction) | |
| is used by guest kernels to transfer control from kernel mode to user | |
| mode. If such an IRET hypercall is placed in the middle of a multicall | |
| batch, subsequent operations invoked by the same multicall batch may | |
| wrongly assume the guest to still be in kernel mode. If one or more of | |
| these subsequent operations involve operations on page tables, they may | |
| be using the wrong root page table, confusing internal accounting. As | |
| a result the guest may gain writable access to some of its page tables. | |
Xen Security Advisory XSA-214 [2]: | |
| grant transfer allows PV guest to elevate privileges | |
| | |
| The GNTTABOP_transfer operation allows one guest to transfer a page to | |
| another guest. The internal processing of this, however, does not | |
| include zapping the previous type of the page being transferred. This | |
| makes it possible for a PV guest to transfer a page previously used as | |
| part of a segment descriptor table to another guest while retaining the | |
| "contains segment descriptors" property. | |
| | |
| If the destination guest is a PV one of different bitness, it may gain | |
| access to segment descriptors it is not normally allowed to have, like | |
| 64-bit code segments in a 32-bit PV guest. | |
| | |
| If the destination guest is a HVM one, that guest may freely alter the | |
| page contents and then hand the page back to the same or another PV | |
| guest. | |
| | |
| In either case, if the destination PV guest then inserts that page into | |
| one of its own descriptor tables, the page still having the designated | |
| type results in validation of its contents being skipped. | |
The second bug requires cooperation between two VMs of different types, | |
which somewhat limits its applicability. | |
The Xen Security Team also announced a third advisory today: XSA-215 | |
"possible memory corruption via failsafe callback"[3]. | |
| Only x86 systems with physical memory extending to a configuration | |
| dependent boundary (5Tb or 3.5Tb) may be affected. Whether they are | |
| actually affected depends on actual physical memory layout. | |
We believe this bug is extremely unlikely to affect any Qubes users due | |
to the very high hardware requirements (5Tb or 3.5Tb of physical | |
memory). | |
Patching | |
========= | |
Patched packages will be built and uploaded to the security-testing | |
repository shortly after this advisory is published. We have recently | |
implemented and published the details of a new, transparent build | |
infrastructure. [4] In this new infrastructure, the source code for | |
packages is pushed to a public repository, and logs from the build | |
process are also publicly published. However, the Xen security policy | |
does not permit us to make this data public until after the embargo has | |
been lifted. [5] While we have already privately built and tested these | |
packages, we must wait until the embargo has been lifted before | |
transparently building the public packages using our new infrastructure. | |
The specific packages that resolve the problem discussed in this | |
bulletin are as follows: | |
For Qubes 3.2: | |
- Xen packages, version 4.6.5-27 | |
The packages are to be installed in dom0 via the qubes-dom0-update | |
command or via the Qubes VM Manager. | |
A system restart will be required afterwards. | |
If you use Anti Evil Maid, you will need to reseal your secret | |
passphrase to new PCR values, as PCR18 will change due to the new xen.gz | |
binary. | |
These packages will migrate to the current (stable) repository over the | |
coming days after being tested by the community. | |
Commentary | |
=========== | |
XSA-213 is a fatal, reliably exploitable bug in Xen. In the nearly eight-year | |
history of the Qubes OS project, we have become aware of four bugs of this | |
calibre: XSA-148 [12], XSA-182 [13], XSA-212 [14], and now XSA-213. | |
Coincidentally, all of these fatal bugs have been in Xen mechanisms for handling | |
memory virtualization for paravirtualized (PV) VMs. | |
Some might argue that having only four fatal bugs (among other not-that-fatal | |
ones [15]) in 8 years is a reasonably good result, especially compared to other | |
desktop systems. We, however, have been deeply upset by each and every of these | |
bugs. In fact, after we learned of the second of these (XSA-182) 10 months ago, | |
we immediately began working on a way to move away from using PV-based VMs and | |
toward using only hardware-based virtualization (HVM) VMs in Qubes 4.x [6]. | |
The switch from PV to HVM has been a major undertaking and has | |
introduced a delay in the release of Qubes 4.0. This undertaking is now | |
complete, however [7], and Qubes 4.0-rc1 will be released in the next | |
1-2 months, after we've finished up with some remaining minor issues. | |
We originally hoped we could transition to running all Linux VMs in a | |
so-called PVH mode of virtualization, where the I/O emulator is not | |
needed at all, but it turned out the Linux kernel is not quite ready for | |
this. So, in Qubes 4.0, we will use the classic HVM mode, where the I/O | |
emulator is sandboxed within... a PV VM (which is also the case when one | |
runs Windows AppVMs on Qubes 3.x). This makes it possible for an | |
attacker to chain attacks: one for QEMU with another hypothetical for PV | |
virtualization, to break out of a VM. But the good news is that, with | |
the work we have done in 4.0 to transition from PV to HVM, the final | |
step to transition to PVH should be trivial, and we hope to introduce it | |
in 4.1, once the upstream Linux kernel supports it. | |
Since we began working on ditching PV virtualization 10 months ago, two | |
more fatal Xen bugs were published: one last month (XSA 212 [9]) and the | |
one we discuss today (XSA 213). To provide our users with some means of | |
addressing these problems, we've recently introduced "Paranoid Backup | |
Recovery" mode [10], which we believe is a meaningful way for users to | |
recover from potential compromises on Qubes OS. | |
Many readers will undoubtedly continue asking, "If Xen is so buggy, why | |
not ditch it for some other hypervisor?" First, all public hypervisors | |
have security issues, and it's unclear whether Xen is any buggier than | |
the other available options. Second, and most importantly, we don't see | |
any good alternative at this moment. Xen has some unique architectural | |
features, such as support for running network and storage backends | |
within _unprivileged_ VMs, which other, popular VMMs do not. Finally, | |
unlike many research projects, Xen is mature enough to support all sorts | |
of features that are necessary when running on a laptop, such as power | |
management and reasonable compatibility with most BIOSes. | |
In principle, however, Xen is an interchangeable component within the | |
Qubes OS architecture. One day, we might replace it with something else, | |
and thanks to the generalized architecture we introduced in Qubes 3.0 | |
[11] and took even further in Qubes 4.0 (e.g. [16]), Qubes users and | |
admins might not even notice! | |
Credits | |
======== | |
See original Xen Security Advisories: | |
- XSA-213 [1] | |
- XSA-214 [2] | |
References | |
=========== | |
[1] https://xenbits.xen.org/xsa/advisory-213.html | |
[2] https://xenbits.xen.org/xsa/advisory-214.html | |
[3] https://xenbits.xen.org/xsa/advisory-215.html | |
[4] https://github.com/QubesOS/qubes-infrastructure/ | |
[5] https://www.xenproject.org/security-policy.html | |
[6] https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-024-2016.txt | |
[7] https://github.com/QubesOS/qubes-issues/issues/2185 | |
[8] https://www.qubes-os.org/news/2016/07/21/new-hw-certification-for-q4/ | |
[9] https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-029-2017.txt | |
[10] https://www.qubes-os.org/news/2017/04/26/qubes-compromise-recovery/ | |
[11] https://blog.invisiblethings.org/2015/04/23/qubes-30rc1-and-roadmap.html | |
[12] https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-022-2015.txt | |
[13] https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-024-2016.txt | |
[14] https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-029-2017.txt | |
[15] https://www.qubes-os.org/security/bulletins/ | |
[16] https://www.qubes-os.org/doc/mgmt/ | |
-- | |
The Qubes Security Team | |
https://www.qubes-os.org/security/ |