Xen up to 4.12.x Promotion race condition

Summaryinfo

A vulnerability has been found in Xen up to 4.12.x and classified as critical. This affects an unknown part of the component Promotion Handler. The manipulation leads to race condition. This vulnerability is uniquely identified as CVE-2019-18421. The attack is possible to be carried out remotely. No exploit exists.

Detailsinfo

A vulnerability was found in Xen up to 4.12.x (Virtualization Software) and classified as critical. This issue affects an unknown part of the component Promotion Handler. The manipulation with an unknown input leads to a race condition vulnerability. Using CWE to declare the problem leads to CWE-362. The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

The weakness was disclosed 10/31/2019 (oss-sec). It is possible to read the advisory at openwall.com. The identification of this vulnerability is CVE-2019-18421 since 10/24/2019. The exploitation is known to be difficult. The attack may be initiated remotely. A simple authentication is necessary for exploitation. The technical details are unknown and an exploit is not publicly available. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 02/01/2024).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

The entries VDB-144711, VDB-144710, VDB-144709 and VDB-144708 are pretty similar. Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Type

• Virtualization Software

Name

• Xen

Version

• 4.0
• 4.1
• 4.2
• 4.3
• 4.4
• 4.5
• 4.6
• 4.7
• 4.8
• 4.9
• 4.10
• 4.11
• 4.12

License

• open-source

CPE 2.3info

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CPE 2.2info

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Entryinfo

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