CVE-2019-3887

Published: 9 April 2019

A flaw was found in the way KVM hypervisor handled x2APIC Machine Specific Rregister (MSR) access with nested(=1) virtualization enabled. In that, L1 guest could access L0's APIC register values via L2 guest, when 'virtualize x2APIC mode' is enabled. A guest could use this flaw to potentially crash the host kernel resulting in DoS issue. Kernel versions from 4.16 and newer are vulnerable to this issue.

From the Ubuntu Security Team

Marc Orr discovered that the KVM hypervisor implementation in the Linux kernel did not properly restrict APIC MSR register values when nested virtualization is used. An attacker in a guest vm could use this to cause a denial of service (host OS crash).

Notes

Ubuntu kernels do not enable nested KVM virtualization by default and
are unaffected by this flaw in the default configuration. However, installing
QEMU results in nested KVM support to be enabled via the
/etc/modprobe.d/qemu-system-x86.conf file. To ensure that nested
virtualization is not enabled, verify that the
/sys/module/kvm_intel/parameters/nested file contains "N".

Mitigation

If qemu is installed, to prevent nested virtualization from being enabled,
set "options kvm_intel nested=0" in /etc/modprobe.d/qemu-system-x86.conf

Status

Severity score breakdown

References

• https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-3887
• https://www.openwall.com/lists/oss-security/2019/04/08/1
• https://ubuntu.com/security/notices/USN-3979-1
• https://ubuntu.com/security/notices/USN-3980-1
• https://ubuntu.com/security/notices/USN-3980-2
• NVD
• Launchpad
• Debian