virt: acrn: Annotate struct vm_memory_region_batch with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct vm_memory_region_batch.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Fei Li <fei1.li@intel.com>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230922175102.work.020-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: kees

drivers/virt/acrn/acrn_drv.h

@@ -60,7 +60,7 @@ struct vm_memory_region_batch {
 	u16			   reserved[3];
 	u32			   regions_num;
 	u64			   regions_gpa;
-	struct vm_memory_region_op regions_op[];
+	struct vm_memory_region_op regions_op[] __counted_by(regions_num);
 };
 
 /**

drivers/virt/acrn/mm.c

@@ -250,11 +250,11 @@ int acrn_vm_ram_map(struct acrn_vm *vm, struct acrn_vm_memmap *memmap)
 		ret = -ENOMEM;
 		goto unmap_kernel_map;
 	}
+	regions_info->regions_num = nr_regions;
 
 	/* Fill each vm_memory_region_op */
 	vm_region = regions_info->regions_op;
 	regions_info->vmid = vm->vmid;
-	regions_info->regions_num = nr_regions;
 	regions_info->regions_gpa = virt_to_phys(vm_region);
 	user_vm_pa = memmap->user_vm_pa;
 	i = 0;