KVM: arm64: vgic-its: Fix L2 entry validation for indirect tables

When checking that the storage address of a device entry is valid,
it is critical to compute the actual address of the entry, rather
than relying on the beginning of the page to match a CPU page of
the same size: for example, if the guest places the table at the
last 64kB boundary of RAM, but RAM size isn't a multiple of 64kB...

Fix this by computing the actual offset of the device ID in the
L2 page, and check the corresponding GFN.
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

virt/kvm/arm/vgic/vgic-its.c

@@ -727,7 +727,12 @@ static bool vgic_its_check_device_id(struct kvm *kvm, struct vgic_its *its,
 	 * Any address beyond our supported 48 bits of PA will be caught
 	 * by the actual check in the final step.
 	 */
-	gfn = (indirect_ptr & GENMASK_ULL(51, 16)) >> PAGE_SHIFT;
+	indirect_ptr &= GENMASK_ULL(51, 16);
+
+	/* Find the address of the actual entry */
+	index = device_id % (SZ_64K / GITS_BASER_ENTRY_SIZE(r));
+	indirect_ptr += index * GITS_BASER_ENTRY_SIZE(r);
+	gfn = indirect_ptr >> PAGE_SHIFT;
 
 	return kvm_is_visible_gfn(kvm, gfn);
 }