Integer underflow in Linux kernel - CVE-2026-63940

Vulnerability identifier: #VU138710

CSH Severity: Low

CVSS v4: 6.8 [CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N]

CVE-ID: CVE-2026-63940

CWE-ID: CWE-191

Exploitation vector: Local access

Exploit availability: No public exploit available

The vulnerability allows a local user to cause a denial of service.

The vulnerability exists due to an integer underflow in KVM SEV Port I/O handling in arch/x86/kvm/svm/sev.c when processing Port I/O requests with a length or count of 0. A local user can trigger a zero-length Port I/O request to cause a denial of service.

The issue affects the handling of VMGEXIT and string I/O requests in the KVM SEV implementation.

Linux kernel
openEuler
Ubuntu
kernel-debuginfo
perf-debuginfo
perf
kernel-tools-devel
kernel-tools-debuginfo
kernel-tools
kernel-source
kernel-headers
kernel-devel
kernel-debugsource
bpftool-debuginfo
bpftool
kernel
python3-perf
python3-perf-debuginfo
linux-ibm (Ubuntu package)
linux-aws (Ubuntu package)
linux-azure (Ubuntu package)
linux-nvidia (Ubuntu package)
linux-nvidia-bos (Ubuntu package)

Install security update from vendor's repository.

Linux kernel - update to 7.0 rc3
kernel-debuginfo - update to 5.10.0-328.0.0.229
perf-debuginfo - update to 5.10.0-328.0.0.229
perf - update to 5.10.0-328.0.0.229
kernel-tools-devel - update to 5.10.0-328.0.0.229
kernel-tools-debuginfo - update to 5.10.0-328.0.0.229
kernel-tools - update to 5.10.0-328.0.0.229
kernel-source - update to 5.10.0-328.0.0.229
kernel-headers - update to 5.10.0-328.0.0.229
kernel-devel - update to 5.10.0-328.0.0.229
kernel-debugsource - update to 5.10.0-328.0.0.229
bpftool-debuginfo - update to 5.10.0-328.0.0.229
bpftool - update to 5.10.0-328.0.0.229
kernel - update to 5.10.0-328.0.0.229
python3-perf - update to 5.10.0-328.0.0.229
python3-perf-debuginfo - update to 5.10.0-328.0.0.229
linux-ibm (Ubuntu package) - addressed in versions 7.0.0-1008.8, 7.0.0-1010.10
linux-aws (Ubuntu package) - addressed in versions 7.0.0-1009.9, 7.0.0-1015.15
linux-azure (Ubuntu package) - addressed in versions 7.0.0-1009.9, 7.0.0-1010.10
linux-nvidia (Ubuntu package) - addressed in versions 7.0.0-1016.16, 7.0.0-1016.16~24.04.1
linux-nvidia-bos (Ubuntu package) - update to 7.0.0-2016.16

• https://git.kernel.org/stable/c/2254972d4d69e279ba4e87bf0968eb08ad0d3c92
• https://git.kernel.org/stable/c/3988bd2723de407ae90fa7a6f6029b4e60238c58
• https://git.kernel.org/stable/c/3b6035bc6bff20e89752ce4358bc4c9a9d5883f2
• https://git.kernel.org/stable/c/c30cde934c7813b4e3069765dac64ce3d31e34f2

• Integer underflow in Linux kernel kvm svm
• Ubuntu update for linux-ibm
• Ubuntu update for linux-azure
• Ubuntu update for linux-aws
• openEuler 22.03 LTS SP4 update for kernel
• Ubuntu update for linux-nvidia-bos
• Ubuntu update for linux-nvidia