CVE-2021-29657

A flaw was found in the Linux kernel. A KVM guest on AMD can launch a nested guest without the Intercept VMRUN control bit by exploiting a TOCTOU vulnerability in nested_svm_vmrun. A malicious guest could use this flaw to gain unrestricted access to host MSRs, possibly leading to guest-to-host escape scenario.

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