UPSTREAM: clone3: switch to copy_struct_from_user()

Switch clone3() syscall from it's own copying struct clone_args from
userspace to the new dedicated copy_struct_from_user() helper.

The change is very straightforward, and helps unify the syscall
interface for struct-from-userspace syscalls. Additionally, explicitly
define CLONE_ARGS_SIZE_VER0 to match the other users of the
struct-extension pattern.

Change-Id: Ib09f7bf0cdfed775b606fb7f1698e12515cd0a6b
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
[christian.brauner@ubuntu.com: improve commit message]
Link: https://lore.kernel.org/r/20191001011055.19283-3-cyphar@cyphar.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

kernel/fork.c

@@ -2561,40 +2561,20 @@ SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
 #ifdef __ARCH_WANT_SYS_CLONE3
 noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
 					      struct clone_args __user *uargs,
-					      size_t size)
+					      size_t usize)
 {
+	int err;
 	struct clone_args args;
 	pid_t *kset_tid = kargs->set_tid;
 
-	if (unlikely(size > PAGE_SIZE))
+	if (unlikely(usize > PAGE_SIZE))
 		return -E2BIG;
-
+	if (unlikely(usize < CLONE_ARGS_SIZE_VER0))
-	if (unlikely(size < sizeof(struct clone_args)))
 		return -EINVAL;
 
-	if (unlikely(!access_ok(VERIFY_READ, uargs, size)))
+	err = copy_struct_from_user(&args, sizeof(args), uargs, usize);
-		return -EFAULT;
+	if (err)
-
+		return err;
-	if (size > sizeof(struct clone_args)) {
-		unsigned char __user *addr;
-		unsigned char __user *end;
-		unsigned char val;
-
-		addr = (void __user *)uargs + sizeof(struct clone_args);
-		end = (void __user *)uargs + size;
-
-		for (; addr < end; addr++) {
-			if (get_user(val, addr))
-				return -EFAULT;
-			if (val)
-				return -E2BIG;
-		}
-
-		size = sizeof(struct clone_args);
-	}
-
-	if (copy_from_user(&args, uargs, size))
-		return -EFAULT;
 
 	if (unlikely(args.set_tid_size > MAX_PID_NS_LEVEL))
 		return -EINVAL;