CVE-2019-18421: An issue was discovered in Xen through 4.12.x allowing x86 PV guest OS users to gain host OS privileges by leveraging race conditions in pagetable pro...

Description

An issue was discovered in Xen through 4.12.x allowing x86 PV guest OS users to gain host OS privileges by leveraging race conditions in pagetable promotion and demotion operations. There are issues with restartable PV type change operations. To avoid using shadow pagetables for PV guests, Xen exposes the actual hardware pagetables to the guest. In order to prevent the guest from modifying these page tables directly, Xen keeps track of how pages are used using a type system; pages must be "promoted" before being used as a pagetable, and "demoted" before being used for any other type. Xen also allows for "recursive" promotions: i.e., an operating system promoting a page to an L4 pagetable may end up causing pages to be promoted to L3s, which may in turn cause pages to be promoted to L2s, and so on. These operations may take an arbitrarily large amount of time, and so must be re-startable. Unfortunately, making recursive pagetable promotion and demotion operations restartable is incredibly complicated, and the code contains several races which, if triggered, can cause Xen to drop or retain extra type counts, potentially allowing guests to get write access to in-use pagetables. A malicious PV guest administrator may be able to escalate their privilege to that of the host. All x86 systems with untrusted PV guests are vulnerable. HVM and PVH guests cannot exercise this vulnerability.

Metrics

CVSS 3.1

7.5/10

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

EPSS Probability

1.68%

74.0th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-362

Affected Software

Vendor	Product	Versions
Xen	Xen	<= 4.12.1
Debian	Debian Linux	9.0
Debian	Debian Linux	10.0
Fedoraproject	Fedora	29
Fedoraproject	Fedora	30
Fedoraproject	Fedora	31
Opensuse	Leap	15.0

References

http://lists.opensuse.org/opensuse-security-announce/2019-11/msg00037.htmlMailing List, Third Party Advisory
http://www.openwall.com/lists/oss-security/2019/10/31/3Mailing List, Patch, Third Party Advisory
http://xenbits.xen.org/xsa/advisory-299.htmlPatch, Vendor Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/2BQKX7M2RHCWDBKNPX4KEBI3MJIH6AYZ/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/I5WWPW4BSZDDW7VHU427XTVXV7ROOFFW/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/IZYATWNUGHRBG6I3TC24YHP5Y3J7I6KH/
https://seclists.org/bugtraq/2020/Jan/21Mailing List, Third Party Advisory
https://security.gentoo.org/glsa/202003-56Third Party Advisory
https://www.debian.org/security/2020/dsa-4602Third Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2019-11/msg00037.htmlMailing List, Third Party Advisory
http://www.openwall.com/lists/oss-security/2019/10/31/3Mailing List, Patch, Third Party Advisory
http://xenbits.xen.org/xsa/advisory-299.htmlPatch, Vendor Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/2BQKX7M2RHCWDBKNPX4KEBI3MJIH6AYZ/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/I5WWPW4BSZDDW7VHU427XTVXV7ROOFFW/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/IZYATWNUGHRBG6I3TC24YHP5Y3J7I6KH/
https://seclists.org/bugtraq/2020/Jan/21Mailing List, Third Party Advisory
https://security.gentoo.org/glsa/202003-56Third Party Advisory
https://www.debian.org/security/2020/dsa-4602Third Party Advisory

Timeline

Published: Oct 31, 2019
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2019-18421?

An issue was discovered in Xen through 4.12.x allowing x86 PV guest OS users to gain host OS privileges by leveraging race conditions in pagetable promotion and demotion operations. There are issues with restartable PV type change operations. To avoid using shadow pagetables for PV guests, Xen exposes the actual hardware pagetables to the guest. In order to prevent the guest from modifying these page tables directly, Xen keeps track of how pages are used using a type system; pages must be "promoted" before being used as a pagetable, and "demoted" before being used for any other type. Xen also allows for "recursive" promotions: i.e., an operating system promoting a page to an L4 pagetable may end up causing pages to be promoted to L3s, which may in turn cause pages to be promoted to L2s, and so on. These operations may take an arbitrarily large amount of time, and so must be re-startable. Unfortunately, making recursive pagetable promotion and demotion operations restartable is incredibly complicated, and the code contains several races which, if triggered, can cause Xen to drop or retain extra type counts, potentially allowing guests to get write access to in-use pagetables. A malicious PV guest administrator may be able to escalate their privilege to that of the host. All x86 systems with untrusted PV guests are vulnerable. HVM and PVH guests cannot exercise this vulnerability.

How severe is CVE-2019-18421?

CVE-2019-18421 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 1.68% probability of exploitation in the next 30 days.

How do I fix CVE-2019-18421?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.