High [CVE-2026-63939] Compute the correct max length of the in-GHCB scratch area

In the Linux kernel, the following vulnerability has been resolved: KVM: SEV: Compute the correct max length of the in-GHCB scratch area When setting the length of the GHCB scratch area, and the area is in the GHCB shared buffer, set the effective length of the scratch area to the max possible size given the start of the guest-provided pointer, and the end of the shared buffer.

The code was "fine" when first introduced, as KVM doesn't consult the length of the buffer when emulating MMIO, because the passed in @len always specifies the *max* size required.

But for PSC requests, the incoming @len is just the minimum length (to process the header), and KVM needs to know the full size of the scratch area to avoid buffer overflows (spoiler alert). Opportunistically rename @len => @min_len to better reflect its role.

A flaw was found in the Linux kernel's Kernel-based Virtual Machine (KVM) with Secure Encrypted Virtualization (SEV) support. This vulnerability arises from an incorrect calculation of the maximum length for the in-Guest-Host Communication Block (GHCB) scratch area.

When processing Page State Change (PSC) requests, KVM uses a minimum length instead of the full required size, which can lead to a buffer overflow. A malicious guest operating system could potentially exploit this to cause a denial of service or escalate privileges on the host system.