virt: vmgenid: remap memory as decrypted

It was found that AWS SEV-SNP enabled instances are not able to boot with
commit 81256a5 ("x86/mm: Make memremap(MEMREMAP_WB) map memory as
encrypted by default") applied and the reason seems to be the vmgenid
device which requires unencrypted writeable memory.

A similar problem was previously fixed in DRM with commit
7dfede7 ("drm/vmwgfx: Fix guests running with TDX/SEV").

Note, trusting vmgenid device in a Confidential VM is questionable: the
malicious host may intentionally avoid notifying the guest when a copy is
created.

Fixes: 81256a5 ("x86/mm: Make memremap(MEMREMAP_WB) map memory as encrypted by default")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: stable@vger.kernel.org # 6.15+
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
(cherry picked from commit 1b48c13)

Author: vittyvk

drivers/virt/vmgenid.c

@@ -75,7 +75,8 @@ static int vmgenid_add_acpi(struct device *dev, struct vmgenid_state *state)
 	phys_addr = (obj->package.elements[0].integer.value << 0) |
 		    (obj->package.elements[1].integer.value << 32);
 
-	virt_addr = devm_memremap(&device->dev, phys_addr, VMGENID_SIZE, MEMREMAP_WB);
+	virt_addr = devm_memremap(&device->dev, phys_addr, VMGENID_SIZE,
+				  MEMREMAP_WB | MEMREMAP_DEC);
 	if (IS_ERR(virt_addr)) {
 		ret = PTR_ERR(virt_addr);
 		goto out;