exec: Set the point of no return sooner

Make the code more robust by marking the point of no return sooner.
This ensures that future code changes don't need to worry about how
they return errors if they are past this point.

This results in no actual change in behavior as __do_execve_file does
not force SIGSEGV when there is a pending fatal signal pending past
the point of no return.  Further the only error returns from de_thread
and exec_mmap that can occur result in fatal signals being pending.
Reviewed-by: NKees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/87sgga5klu.fsf_-_@x220.int.ebiederm.orgSigned-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

fs/exec.c

@@ -1304,6 +1304,11 @@ int begin_new_exec(struct linux_binprm * bprm)
 	struct task_struct *me = current;
 	int retval;
 
+	/*
+	 * Ensure all future errors are fatal.
+	 */
+	bprm->point_of_no_return = true;
+
 	/*
 	 * Make this the only thread in the thread group.
 	 */
@@ -1326,13 +1331,6 @@ int begin_new_exec(struct linux_binprm * bprm)
 	if (retval)
 		goto out;
 
-	/*
-	 * With the new mm installed it is completely impossible to
-	 * fail and return to the original process.  If anything from
-	 * here on returns an error, the check in __do_execve_file()
-	 * will SEGV current.
-	 */
-	bprm->point_of_no_return = true;
 	bprm->mm = NULL;
 
 #ifdef CONFIG_POSIX_TIMERS