CVE-2025-38508
HighCVSS 7.3Exploitation Probability (EPSS)
Low risk3th percentile - higher than 3% of all known CVEs
Summary
In the Linux kernel for x86/SEV, a bug was found in Secure TSC frequency calculation. Using the nominal P0 frequency from the GUEST_TSC_FREQ MSR causes a slight deviation from the actual mean TSC frequency, leading to accumulating clock skew between the hypervisor and SEV-SNP VM. The fix uses TSC_FACTOR from the SEV firmware secrets page to accurately calculate the mean TSC frequency.
Risk Assessment
The organization may experience inaccurate timekeeping in SEV-SNP virtual machines, leading to premature timer interrupts and potential application synchronization issues.
Recommendation
Immediately update the Linux kernel to a version containing this fix to ensure accurate timekeeping in SEV-SNP environments.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: x86/sev: Use TSC_FACTOR for Secure TSC frequency calculation When using Secure TSC, the GUEST_TSC_FREQ MSR reports a frequency based on the nominal P0 frequency, which deviates slightly (typically ~0.2%) from the actual mean TSC frequency due to clocking parameters. Over extended VM uptime, this discrepancy accumulates, causing clock skew between the hypervisor and a SEV-SNP VM, leading to early timer interrupts as perceived by the guest. The guest kernel relies on the reported nominal frequency for TSC-based timekeeping, while the actual frequency set during SNP_LAUNCH_START may differ. This mismatch results in inaccurate time calculations, causing the guest to perceive hrtimers as firing earlier than expected. Utilize the TSC_FACTOR from the SEV firmware's secrets page (see "Secrets Page Format" in the SNP Firmware ABI Specification) to calculate the mean TSC frequency, ensuring accurate timekeeping and mitigating clock skew in SEV-SNP VMs. Use early_ioremap_encrypted() to map the secrets page as ioremap_encrypted() uses kmalloc() which is not available during early TSC initialization and causes a panic. [ bp: Drop the silly dummy var: https://lore.kernel.org/r/20250630192726.GBaGLlHl84xIopx4Pt@fat_crate.local ]

