CVE-2021-3656

Published: 16 August 2021

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.

From the Ubuntu Security Team

Maxim Levitsky and Paolo Bonzini discovered that the KVM hypervisor implementation for AMD processors in the Linux kernel allowed a guest VM to disable restrictions on VMLOAD/VMSAVE in a nested guest. An attacker in a guest VM could use this to read or write portions of the host's physical memory.

Mitigation

Disable nested virtualisation when loading the KVM AMD module:
   modprobe kvm_amd nested=0

Status

Severity score breakdown

References

• https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3656
• https://github.com/torvalds/linux/commit/89c8a4984fc9
• https://www.openwall.com/lists/oss-security/2021/08/16/1
• https://ubuntu.com/security/notices/USN-5070-1
• https://ubuntu.com/security/notices/USN-5071-1
• https://ubuntu.com/security/notices/USN-5072-1
• https://ubuntu.com/security/notices/USN-5073-1
• https://ubuntu.com/security/notices/USN-5071-2
• https://ubuntu.com/security/notices/USN-5082-1
• https://ubuntu.com/security/notices/USN-5073-2
• NVD
• Launchpad
• Debian