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.
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (1.68%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
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.
- CVSS 3.1
- 7.5 HIGHCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
- EPSS
- 1.68% probability · 76th percentile
- CISA KEV
- Not listed
- Weakness
- CWE-362
- Affected
- xen/xen · debian/debian linux · fedoraproject/fedora · opensuse/leap
- Source
- cve@mitre.org
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
Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.