CVE-2026-63941
HighCVSS 8.8Exploitation Probability (EPSS)
Low risk5th percentile - higher than 5% of all known CVEs
Summary
A vulnerability was found in the Linux kernel's KVM for ARM64, related to incorrect capping of the ZCR_EL2 register by a guest hypervisor. The issue occurs when the guest updates ZCR_EL2 via ZCR_EL1 without proper sanitization, allowing L2 guests to access unauthorized vector lengths (VL).
Risk Assessment
The risk is that a guest hypervisor could access larger SVE vector lengths than permitted, potentially breaking isolation between virtual machines and leading to data leakage.
Recommendation
It is recommended to immediately update the Linux kernel to a version containing the fix, which moves VL capping to SVE context restore points and marks ZCR_EL2 as a sanitized register.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: KVM: arm64: Correctly cap ZCR_EL2 provided by a guest hypervisor ZCR_EL2 can be updated by a VHE guest hypervisor either using ZCR_EL2 (which traps) or ZCR_EL1 (which does not trap). KVM handles both in different way: - on ZCR_EL2 trap, ZCR_EL2.LEN is immediately capped at the VM's own VL limit. This has the potential to break existing SW that relies on the full LEN field to be stateful. - on ZCR_EL1 access, we do absolutely nothing. On restoring the SVE context for an L2 guest, we directly restore the guest hypervisor's view of ZCR_EL2 into the physical ZCR_EL2. If the guest's view of the register was updated using the ZCR_EL2 accessor, the value has already been sanitised (with the caveat mentioned above). But if the guest used ZCR_EL1, the raw value is written into the HW, and the L2 guest can now access VLs that it shouldn't. Fix all the above by moving the VL capping to the restore points, ensuring that: - the HW is always programmed with a capped value, irrespective of the accessor being used, - the ZCR_EL2.LEN field is always completely stateful, irrespective of the accessor being used. Additionally, move ZCR_EL2 to be a sanitised register, ensuring that only the LEN field is actually stateful. This requires some creative construction of the RES0 mask, as the sysreg generation script does not yet generate RAZ/WI fields. [maz: rewrote commit message, tidy up access_zcr_el2()]