Merge tag 'for-linus-20190912' of gitolite.kernel.org:pub/scm/linux/kernel/git/brauner/linux

Pull clone3 fix from Christian Brauner:
 "This is a last-minute bugfix for clone3() that should go in before we
  release 5.3 with clone3().

  clone3() did not verify that the exit_signal argument was set to a
  valid signal. This can be used to cause a crash by specifying a signal
  greater than NSIG. e.g. -1.

  The commit from Eugene adds a check to copy_clone_args_from_user() to
  verify that the exit signal is limited by CSIGNAL as with legacy
  clone() and that the signal is valid. With this we don't get the
  legacy clone behavior were an invalid signal could be handed down and
  would only be detected and then ignored in do_notify_parent(). Users
  of clone3() will now get a proper error right when they pass an
  invalid exit signal. Note, that this is not a change in user-visible
  behavior since no kernel with clone3() has been released yet"

* tag 'for-linus-20190912' of gitolite.kernel.org:pub/scm/linux/kernel/git/brauner/linux:
  fork: block invalid exit signals with clone3()

kernel/fork.c

@@ -2338,6 +2338,8 @@ struct mm_struct *copy_init_mm(void)
  *
  * It copies the process, and if successful kick-starts
  * it and waits for it to finish using the VM if required.
+ *
+ * args->exit_signal is expected to be checked for sanity by the caller.
  */
 long _do_fork(struct kernel_clone_args *args)
 {
@@ -2562,6 +2564,14 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
 	if (copy_from_user(&args, uargs, size))
 		return -EFAULT;
 
+	/*
+	 * Verify that higher 32bits of exit_signal are unset and that
+	 * it is a valid signal
+	 */
+	if (unlikely((args.exit_signal & ~((u64)CSIGNAL)) ||
+		     !valid_signal(args.exit_signal)))
+		return -EINVAL;
+
 	*kargs = (struct kernel_clone_args){
 		.flags		= args.flags,
 		.pidfd		= u64_to_user_ptr(args.pidfd),