Time-of-check Time-of-use (TOCTOU) Race Condition in Linux kernel - CVE-2026-63937

Vulnerability identifier: #VU138707

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-2026-63937

CWE-ID: CWE-367

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 a time-of-check time-of-use race condition in the KVM SEV Page State Change buffer handling in arch/x86/kvm/svm/sev.c when processing guest-accessible PSC buffer entries and indices. A local user can modify buffer values during processing to cause a denial of service.

The issue involves guest-accessible PSC buffer header fields and entries used for validation and array indexing.

Linux kernel
Ubuntu
linux-ibm (Ubuntu package)
linux-azure (Ubuntu package)
linux-aws (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
linux-ibm (Ubuntu package) - addressed in versions 7.0.0-1008.8, 7.0.0-1010.10
linux-azure (Ubuntu package) - addressed in versions 7.0.0-1009.9, 7.0.0-1010.10
linux-aws (Ubuntu package) - addressed in versions 7.0.0-1009.9, 7.0.0-1015.15
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/b1dfaa6f7a957726a6800135be3659fbe4bbf2a4
• https://git.kernel.org/stable/c/bd232801ef1d1fd985d2d4ca3cd1d888303ca86f
• https://git.kernel.org/stable/c/c8cc238093ca6c99267032f6cfe78f59389f3157
• https://git.kernel.org/stable/c/edbbe88f83b524434974e84808d3093199d67c24

• Time-of-check Time-of-use (TOCTOU) Race Condition in Linux kernel kvm svm
• Ubuntu update for linux-ibm
• Ubuntu update for linux-azure
• Ubuntu update for linux-aws
• Ubuntu update for linux-nvidia-bos
• Ubuntu update for linux-nvidia