CVE-2021-28038: 
Linux Kernel vulnerability analysis and mitigation

An issue was discovered in the Linux kernel through 5.11.3, as used with Xen PV. The vulnerability (CVE-2021-28038) was discovered on March 5, 2021, and involves the netback driver lacking necessary treatment of errors such as failed memory allocations. This vulnerability exists as a result of an incomplete fix for CVE-2021-26931. The issue affects Linux versions from at least 2.6.39 onwards when run in PV mode (Xen Advisory).

The vulnerability stems from insufficient error handling in the netback driver's grant mapping functionality. The issue specifically relates to changes in the handling of grant mapping errors, where a certain function invocation was left with effectively no error handling. This resulted in memory allocation failures not being properly handled. The vulnerability has been assigned a CVSS v3.1 Base Score of 6.5 (MEDIUM) with the vector string CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H (NVD).

A malicious or buggy networking frontend driver may be able to crash the corresponding backend driver, potentially affecting the entire domain running the backend driver. In a typical (non-disaggregated) system, this results in a host-wide denial of service (DoS). The vulnerability specifically impacts systems running Linux in PV mode, while Linux running in HVM/PVH modes is not affected (Xen Advisory).

The vulnerability can be exploited by a malicious guest to cause a denial of service (crash) within the domain running the backend, which is typically dom0. The issue was reported by Intel's kernel test robot and recognized as a security issue by Jan Beulich of SUSE. The vulnerability was reported publicly before an advisory could be issued (Xen Advisory).

For Linux systems, running the backends in HVM or PVH domains will avoid the vulnerability. This can be achieved by running the dom0 in PVH mode. In all other cases, there is no known mitigation. A patch has been released to resolve the issue, which properly handles the grant mapping errors (Xen Advisory).