CVE-2026-63937: KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer

In the Linux kernel, the following vulnerability has been resolved:

KVM: SEV: Use READONCE() when reading entries/indices from PSC buffer

Use READONCE() when reading entries/indices from the guest-accessible Page State Change buffer to defend against TOCTOU bugs.

Don't bother with READONCE()/WRITEONCE() for cases where KVM is writing (and not consuming the result!), as the guest isn't supposed to touch the buffer while it's being processed. I.e. using READONCE() is all about protecting against misbehaving guests.