CVE-2026-63939

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.