New Linux Kernel Flaw Gives ARM64 KVM Guests Read-Write Access to Host Memory
A vulnerability in the Linux kernel's KVM virtualization code for ARM64 processors allows a guest virtual machine to read and write host kernel memory, potentially leading to an attacker gaining control of the underlying host machine. While no exploit code has been released, the flaw could be used to break into systems with nested virtualization enabled, particularly on platforms like Red Hat Enterprise Linux. Patches are available in Linux 6.18.51, 7.2.5, and 7.3-rc1.
A new vulnerability, CVE-2026-89775, exists within the Linux kernel's KVM virtualization code for ARM64 processors, presenting a significant risk to systems utilizing nested virtualization. This flaw enables a guest virtual machine to access and modify host kernel memory, potentially allowing an attacker to execute code on the underlying host machine. The vulnerability stems from a specific memory management issue within KVM, where a size calculation can result in a step intended to clear stale entries from the processor's address cache being skipped, leaving a page of host memory mapped and writable for the guest to access.
Hyunwoo Kim, the security researcher who discovered the flaw, has previously identified similar KVM guest-to-host escapes, including ITScape on ARM64, and Januscape and Zapscape on x86. The vulnerability is particularly concerning on systems with nested virtualization enabled, which is not the default on ARM64. Red Hat Enterprise Linux lists its version 10 kernel as affected, while versions 6 through 9 are not.
On systems where a user can access /dev/kvm, a local user could build a guest and exploit the same bug to gain root access. Major cloud providers like Amazon Web Services and Google Cloud have taken steps to mitigate the risk, as their ARM offerings do not typically support nested virtualization. The vulnerability is scored between 7.8 and 9.3 by vendors, reflecting the difficulty of exploitation and the local nature of the attack. As of September 22, the U.S. CISA catalog did not list the vulnerability. Patches are available in Linux 6.18.51, 7.2.5, and 7.3-rc1, and distributions are releasing them at their own pace. The potential impact raises questions about cloud tenants and their ability to exploit this vulnerability on provider machines.