drm/nouveau/nvif: Avoid build error due to potential integer overflows

Trying to build parisc:allmodconfig with gcc 12.x or later results
in the following build error.

drivers/gpu/drm/nouveau/nvif/object.c: In function 'nvif_object_mthd':
drivers/gpu/drm/nouveau/nvif/object.c:161:9: error:
'memcpy' accessing 4294967264 or more bytes at offsets 0 and 32 overlaps 6442450881 bytes at offset -2147483617 [-Werror=restrict]
161 | memcpy(data, args->mthd.data, size);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/nouveau/nvif/object.c: In function 'nvif_object_ctor':
drivers/gpu/drm/nouveau/nvif/object.c:298:17: error:
'memcpy' accessing 4294967240 or more bytes at offsets 0 and 56 overlaps 6442450833 bytes at offset -2147483593 [-Werror=restrict]
298 | memcpy(data, args->new.data, size);

gcc assumes that 'sizeof(*args) + size' can overflow, which would result
in the problem.

The problem is not new, only it is now no longer a warning but an error
since W=1 has been enabled for the drm subsystem and since Werror is
enabled for test builds.

Rearrange arithmetic and use check_add_overflow() for validating the
allocation size to avoid the overflow. While at it, split assignments
out of if conditions.

Fixes: a61ddb4393ad ("drm: enable (most) W=1 warnings by default across the subsystem")
Cc: Javier Martinez Canillas <[e-mail]>
Cc: Jani Nikula <[e-mail]>
Cc: Thomas Zimmermann <[e-mail]>
Cc: Danilo Krummrich <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Christophe JAILLET <[e-mail]>
Cc: Joe Perches <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Guenter Roeck <[e-mail]>
Signed-off-by: Danilo Krummrich <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/nouveau/nvif/object.c

···
 		struct nvif_ioctl_v0 ioctl;
 		struct nvif_ioctl_mthd_v0 mthd;
 	} *args;
+	u32 args_size;
 	u8 stack[128];
 	int ret;
 
-	if (sizeof(*args) + size > sizeof(stack)) {
-		if (!(args = kmalloc(sizeof(*args) + size, GFP_KERNEL)))
+	if (check_add_overflow(sizeof(*args), size, &args_size))
+		return -ENOMEM;
+
+	if (args_size > sizeof(stack)) {
+		args = kmalloc(args_size, GFP_KERNEL);
+		if (!args)
 			return -ENOMEM;
 	} else {
 		args = (void *)stack;
···
 	args->mthd.method = mthd;
 
 	memcpy(args->mthd.data, data, size);
-	ret = nvif_object_ioctl(object, args, sizeof(*args) + size, NULL);
+	ret = nvif_object_ioctl(object, args, args_size, NULL);
 	memcpy(data, args->mthd.data, size);
 	if (args != (void *)stack)
 		kfree(args);
···
 	object->map.size = 0;
 
 	if (parent) {
-		if (!(args = kmalloc(sizeof(*args) + size, GFP_KERNEL))) {
+		u32 args_size;
+
+		if (check_add_overflow(sizeof(*args), size, &args_size)) {
+			nvif_object_dtor(object);
+			return -ENOMEM;
+		}
+
+		args = kmalloc(args_size, GFP_KERNEL);
+		if (!args) {
 			nvif_object_dtor(object);
 			return -ENOMEM;
 		}
···
 		args->new.oclass = oclass;
 
 		memcpy(args->new.data, data, size);
-		ret = nvif_object_ioctl(parent, args, sizeof(*args) + size,
-					&object->priv);
+		ret = nvif_object_ioctl(parent, args, args_size, &object->priv);
 		memcpy(data, args->new.data, size);
 		kfree(args);
 		if (ret == 0)