CVE-2026-63941

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 VMs 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 hypervisors view of ZCR_EL2 into the physical ZCR_EL2. If the guests 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 shouldnt.

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()]

Weakness

The product does not perform an authorization check when an actor attempts to access a resource or perform an action.

Affected Software

Potential Mitigations

• Divide the product into anonymous, normal, privileged, and administrative areas. Reduce the attack surface by carefully mapping roles with data and functionality. Use role-based access control (RBAC) [REF-229] to enforce the roles at the appropriate boundaries.
• Note that this approach may not protect against horizontal authorization, i.e., it will not protect a user from attacking others with the same role.
• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
• For example, consider using authorization frameworks such as the JAAS Authorization Framework [REF-233] and the OWASP ESAPI Access Control feature [REF-45].
• For web applications, make sure that the access control mechanism is enforced correctly at the server side on every page. Users should not be able to access any unauthorized functionality or information by simply requesting direct access to that page.
• One way to do this is to ensure that all pages containing sensitive information are not cached, and that all such pages restrict access to requests that are accompanied by an active and authenticated session token associated with a user who has the required permissions to access that page.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/665.html

References

• https://git.kernel.org/stable/c/2090c1a2368339eab0e14dcde12fdf6a5ea6359d
• https://git.kernel.org/stable/c/742a9b5ccd8c46caf983ca90c94f855466968e34
• https://git.kernel.org/stable/c/83726330748981372bde86ed5411d7b306612991