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 a race condition whereby a malicious PV guest can bypass BTC/SRSO protections and launch a BTC/SRSO attack against Xen.
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Xen | Xen | * | * |
| Xen | Ubuntu | bionic | * |
| Xen | Ubuntu | focal | * |
| Xen | Ubuntu | lunar | * |
| Xen | Ubuntu | mantic | * |
| Xen | Ubuntu | oracular | * |
| Xen | Ubuntu | plucky | * |
| Xen | Ubuntu | trusty | * |
| Xen | Ubuntu | xenial | * |