---===[ Qubes Security Bulletin 093 ]===---

                              2023-08-09

      Transient execution vulnerabilities in AMD and Intel CPUs
           (CVE-2023-20569/XSA-434, CVE-2022-40982/XSA-435)

User action required
---------------------

Users must install the following specific packages in order to address
the issues discussed in this bulletin:

  For Qubes 4.1, in dom0:
  - Xen packages, version 4.14.6-1
  - microcode_ctl, version 2.1-55

  For Qubes 4.2, in dom0:
  - Xen packages, version 4.17.2-1
  - microcode_ctl, version 2.1-55

Note on AMD Zen 1 and Zen 2 CPUs: The packages we previously released
for QSB-086 [1] already contain mitigations that are sufficient to
protect these CPUs from CVE-2023-20569/XSA-434. Consequently,
fully-updated [2] Qubes OS installations running on systems with these
CPUs are not affected by the vulnerabilities discussed in this bulletin.

Note on AMD Zen 3 and Zen 4 CPUs: AMD has stated that they plan to
distribute microcode updates for these CPUs to original equipment
manufacturers (OEMs), original design manufacturers (ODMs), and
motherboard manufacturers (MB). [3] These microcode updates are shipped
only as part of system firmware; loading them from the operating system
is not supported. Therefore, until the relevant OEM, ODM, or MB provides
a suitable BIOS or (U)EFI update for a system, the package updates
listed above will not be sufficient to address CVE-2023-20569/XSA-434 on
that system.

These packages will migrate from the security-testing repository to the
current (stable) repository over the next two weeks after being tested
by the community. [4] Once available, the packages are to be installed
via the Qubes Update tool or its command-line equivalents. [2]

Dom0 must be restarted afterward in order for the updates to take
effect.

If you use Anti Evil Maid, you will need to reseal your secret
passphrase to new PCR values, as PCR18+19 will change due to the new
Xen binaries.

Summary
--------

The Xen Project published the following security advisories on
2023-08-08:

XSA-434 [5] "x86/AMD: Speculative Return Stack Overflow"
(CVE-2023-20569):

| Researchers from ETH Zurich have extended their prior research
| (XSA-422, Branch Type Confusion, a.k.a Retbleed) and have discovered
| INCEPTION, also know as RAS (Return Address Stack) Poisoning, and
| Speculative Return Stack Overflow.
|
| The RAS is updated when a CALL instruction is predicted, rather than
| at a later point in the pipeline.  However, the RAS is still
| fundamentally a circular stack.
|
| It is possible to poison the branch type and target predictions such
| that, at a point of the attackers choosing, the branch predictor
| predicts enough CALLs back-to-back to wrap around the entire RAS and
| overwrite a correct return prediction with one of the attackers
| choosing.
|
| This allows the attacker to control RET speculation in a victim
| context, and leak arbitrary data as a result.
|
| For more details, see:
|   https://comsec.ethz.ch/inception
|   https://www.amd.com/en/corporate/product-security/bulletin/amd-sb-7005

XSA-435 [6] "x86/Intel: Gather Data Sampling" (CVE-2022-40982):

| A researcher has discovered Gather Data Sampling, a transient
| execution side-channel whereby the AVX GATHER instructions can forward
| the content of stale vector registers to dependent instructions.
|
| The physical register file is a structure competitively shared between
| sibling threads.  Therefore an attacker can infer data from the
| sibling thread, or from a more privileged context.
|
| For more details, see:
|   https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/gather-data-sampling.html

Impact
-------

An attacker who compromises one qube can attempt to exploit one of these
vulnerabilities (the one corresponding to the system's CPU) in order to
infer the contents of data belonging to other qubes. In systems with AMD
CPUs, successfully exploiting CVE-2023-20569/XSA-434 would allow an
attacker to infer the contents of arbitrary host memory. In systems with
Intel CPUs, successfully exploiting CVE-2022-40982/XSA-435 would allow
an attacker to infer data from different CPU contexts on the same core.

Credits
--------

See the original Xen Security Advisories.

References
-----------

[1] https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-086-2022.txt
[2] https://www.qubes-os.org/doc/how-to-update/
[3] https://www.amd.com/en/resources/product-security/bulletin/amd-sb-7005.html
[4] https://www.qubes-os.org/doc/testing/
[5] https://xenbits.xen.org/xsa/advisory-434.html
[6] https://xenbits.xen.org/xsa/advisory-435.html

--
The Qubes Security Team
https://www.qubes-os.org/security/