KVM: SEV: Drop WARN on large size for KVM_MEMORY_ENCRYPT_REG_REGION

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

KVM: SEV: Drop WARN on large size for KVM_MEMORY_ENCRYPT_REG_REGION

Drop the WARN in sev_pin_memory() on npages overflowing an int, as the

WARN is comically trivially to trigger from userspace, e.g. by doing:

struct kvm_enc_region range = {

.addr = 0,

.size = -1ul,

};

__vm_ioctl(vm, KVM_MEMORY_ENCRYPT_REG_REGION, &range);

Note, the checks in sev_mem_enc_register_region() that presumably exist to

verify the incoming address+size are completely worthless, as both "addr"

and "size" are u64s and SEV is 64-bit only, i.e. they _can't_ be greater

than ULONG_MAX. That wart will be cleaned up in the near future.

if (range->addr > ULONG_MAX || range->size > ULONG_MAX)

return -EINVAL;

Opportunistically add a comment to explain why the code calculates the

number of pages the "hard" way, e.g. instead of just shifting @ulen.