CVE-2023-46836 Information

Description

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.

Reference

https://xenbits.xenproject.org/xsa/advisory-446.html