CVE-2021-3656

A flaw was found in the KVM's AMD code for supporting SVM nested virtualization. The flaw occurs when processing the VMCB (virtual machine control block) provided by the L1 guest to spawn/handle a nested guest (L2). Due to improper validation of the "virt_ext" field, this issue could allow a malicious L1 to disable both VMLOAD/VMSAVE intercepts and VLS (Virtual VMLOAD/VMSAVE) for the L2 guest. As a result, the L2 guest would be allowed to read/write physical pages of the host, resulting in a crash of the entire system, leak of sensitive data or potential guest-to-host escape.

The nested virtualization feature is not enabled by default up to Red Hat Enterprise Linux 8.4. Most importantly, Red Hat currently provides nested virtualization only as a Technology Preview, and is therefore unsupported for production use. For additional details please see https://access.redhat.com/solutions/21101 and https://access.redhat.com/support/offerings/techpreview.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

This vulnerability can be mitigated by disabling the nested virtualization feature:

# modprobe -r kvm_amd
# modprobe kvm_amd nested=0

Disabling VLS (Virtual VMLOAD/VMSAVE) is an alternative mitigation:

# modprobe kvm_amd vls=0