QSB-108: Transitive Scheduler Attacks (XSA-471)
https://www.qubes-os.org/news/2025/07/11/qsb-108/
We have published Qubes Security Bulletin (QSB) 108: Transitive Scheduler Attacks (XSA-471) ( https://github.com/QubesOS/qubes-secpack/blob/5da25fc8fbece540ad4594235cf7ee5e7a751fbb/QSBs/qsb-108-2025.txt). The text of this QSB and its accompanying cryptographic signatures are reproduced below, followed by a general explanation of this announcement and authentication instructions.
Qubes Security Bulletin 108
---===[ Qubes Security Bulletin 108 ]===---
2025-07-08
Transitive Scheduler Attacks (XSA-471)
Changelog
----------
2025-07-08: Original QSB
2025-07-11: Revise language
User action
------------
Continue to update normally [1] in order to receive the security updates
described in the "Patching" section below. No other user action is
required in response to this QSB.
Summary
--------
On 2025-07-08, the Xen Project published XSA-471, "x86: Transitive
Scheduler Attacks" (CVE-2024-36350, CVE-2024-36357) [3]:
| Researchers from Microsoft and ETH Zurich have discovered several new
| speculative sidechannel attacks which bypass current protections.
| They are detailed in a paper titled "Enter, Exit, Page Fault, Leak:
| Testing Isolation Boundaries for Microarchitectural Leaks".
|
| Two issues, which AMD have named Transitive Scheduler Attacks, utilise
| timing information from instruction execution. These are:
|
| * CVE-2024-36350: TSA-SQ (TSA in the Store Queues)
| * CVE-2024-36357: TSA-L1 (TSA in the L1 data cache)
For more information, see also [4], [5] and [6].
Impact
-------
On affected systems, an attacker who manages to compromise a qube may be
able to use it to infer the contents of arbitrary system memory,
including memory assigned to other qubes.
As noted in XSA-471, the paper [6] also describes two Rogue System
Register Read (sometimes called Spectre-v3a) attacks, namely
CVE-2024-36348 and CVE-2045-36349. However, these are not believed to
affect the security of Qubes OS.
Affected systems
-----------------
Only AMD CPUs with Zen 3 or Zen 4 cores are believed to be affected
(CPUID family 0x19). For a more detailed list, see [5].
Patching
---------
As of this writing, AMD has published only non-server CPU microcode
updates via the linux-firmware repository. [7] They have not yet
published microcode updates for server CPUs. When this happens, we will
provide an updated amd-ucode-firmware package. Users with server CPUs
may be able to obtain the relevant microcode update via a motherboard
firmware (BIOS/UEFI) update, but this depends on the motherboard vendor
making such an update available. The appendix of [4] (page 5) contains a
table showing the minimum microcode version required for mitigating
transient scheduler attacks for different CPUs. The required microcode
version (not to be confused with the amd-ucode-firmware package version)
depends on the CPUID family/model/stepping. Users can compare the
values from the table with their own system's family/model/stepping and
current microcode version, which can be viewed by executing the command
`cat /proc/cpuinfo` in a dom0 terminal.
On affected systems with non-server CPUs, the following packages contain
security updates that address the vulnerabilities described in this
bulletin:
For Qubes 4.2, in dom0:
- Xen packages, version 4.17.5-10
- amd-ucode-firmware version 20250708-1
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. [2] Once available, the packages should be installed
via the Qubes Update tool or its command-line equivalents. [1]
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.
Credits
--------
