insufficient TLB flush for x86 PV guests in shadow mode

2022-07-2610:59:00

Xen Project

xenbits.xen.org

tlb flush

x86 pv guests

shadow mode

dos

vulnerability

xen

xsa-401

CVSS3

8.8

Attack Vector

LOCAL

Attack Complexity

LOW

Privileges Required

LOW

User Interaction

NONE

Scope

CHANGED

Confidentiality Impact

HIGH

Integrity Impact

HIGH

Availability Impact

HIGH

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

EPSS

Percentile

14.2%

JSON

ISSUE DESCRIPTION

For migration as well as to work around kernels unaware of L1TF (see XSA-273), PV guests may be run in shadow paging mode. To address XSA-401, code was moved inside a function in Xen. This code movement missed a variable changing meaning / value between old and new code positions. The now wrong use of the variable did lead to a wrong TLB flush condition, omitting flushes where such are necessary.

IMPACT

The known (observed) impact would be a Denial of Service (DoS) affecting the entire host, due to running out of memory. Privilege escalation and information leaks cannot be ruled out.

VULNERABLE SYSTEMS

All versions of Xen with the XSA-401 fixes applied are vulnerable.
Only x86 PV guests can trigger this vulnerability, and only when running in shadow mode. Shadow mode would be in use when migrating guests or as a workaround for XSA-273 (L1TF).