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A Xen security update has been released for Qubes OS

QSB-096: BTC/SRSO fixes not fully effective (XSA-446)

We have published Qubes Security Bulletin 096: BTC/SRSO fixes not fully effective (XSA-446). The text of this QSB and its accompanying cryptographic signatures are reproduced below. For an explanation of this announcement and instructions for authenticating this QSB, please see the end of this announcement.

Qubes Security Bulletin 096

---===[ Qubes Security Bulletin 096 ]===---


BTC/SRSO fixes not fully effective (XSA-446)

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.


On 2023-11-14, the Xen Project published XSA-446, "x86: BTC/SRSO fixes
not fully effective" [3]:

| The fixes for XSA-422 (Branch Type Confusion) and XSA-434 (Speculative
| Return Stack Overflow) are not IRQ-safe. It was believed that the
| mitigations always operated in contexts with IRQs disabled.
| However, the original XSA-254 fix for Meltdown (XPTI) deliberately
| left interrupts enabled on two entry paths; one unconditionally, and
| one conditionally on whether XPTI was active.
| As BTC/SRSO and Meltdown affect different CPU vendors, the mitigations
| are not active together by default. Therefore, there is race
| condition whereby a malicious PV guest can bypass BTC/SRSO protections
| and launch a BTC/SRSO attack against Xen.


The impact is the same as it was in QSB-086 [4]:

| On Qubes OS installations with affected CPUs, a VM running in PV mode
| may be capable of inferring the memory contents of other running VMs,
| including dom0. In the default Qubes OS configuration, only the
| stubdomains for HVMs are in a position to exploit this vulnerability
| in order to attack other VMs. (Dom0 also runs in PV mode, but it is
| fully trusted.)

Affected systems

Only x86 AMD and Hygon systems are vulnerable.


The following packages contain security updates that address the
vulnerabilities described in this bulletin:

For Qubes 4.1, in dom0:
- Xen packages, version 4.14.6-4

For Qubes 4.2, in dom0:
- Xen packages, version 4.17.2-5

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 are to 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

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.


See the original Xen Security Advisory.


[1] https://www.qubes-os.org/doc/how-to-update/
[2] https://www.qubes-os.org/doc/testing/
[3] https://xenbits.xen.org/xsa/advisory-446.html
[4] https://www.qubes-os.org/news/2022/11/08/qsb-086/

The Qubes Security Team

Source: https://github.com/QubesOS/qubes-secpack/blob/main/QSBs/qsb-096-2023.txt

Marek Marczykowski-Górecki’s PGP signature