exec: Trivial cleanups for exec

This is a continuation of my work to clean up exec so it's more
difficult problems are approachable.

The changes correct some comments, and moves the point_of_no_return
variable up to when the point_of_no_return actually occurs.

Eric W. Biederman (5):
      exec: Move the comment from above de_thread to above unshare_sighand
      exec: Fix spelling of search_binary_handler in a comment
      exec: Run sync_mm_rss before taking exec_update_mutex
      exec: Move handling of the point of no return to the top level
      exec: Set the point of no return sooner

 fs/exec.c | 46 ++++++++++++++++++++++++----------------------
 1 file changed, 24 insertions(+), 22 deletions(-)

Link: https://lkml.kernel.org/r/87sgga6ze4.fsf@x220.int.ebiederm.orgSigned-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

fs/exec.c

@@ -1051,13 +1051,14 @@ static int exec_mmap(struct mm_struct *mm)
 	tsk = current;
 	old_mm = current->mm;
 	exec_mm_release(tsk, old_mm);
+	if (old_mm)
+		sync_mm_rss(old_mm);
 
 	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
 	if (ret)
 		return ret;
 
 	if (old_mm) {
-		sync_mm_rss(old_mm);
 		/*
 		 * Make sure that if there is a core dump in progress
 		 * for the old mm, we get out and die instead of going
@@ -1093,12 +1094,6 @@ static int exec_mmap(struct mm_struct *mm)
 	return 0;
 }
 
-/*
- * This function makes sure the current process has its own signal table,
- * so that flush_signal_handlers can later reset the handlers without
- * disturbing other processes.  (Other processes might share the signal
- * table via the CLONE_SIGHAND option to clone().)
- */
 static int de_thread(struct task_struct *tsk)
 {
 	struct signal_struct *sig = tsk->signal;
@@ -1240,6 +1235,12 @@ static int de_thread(struct task_struct *tsk)
 }
 
 
+/*
+ * This function makes sure the current process has its own signal table,
+ * so that flush_signal_handlers can later reset the handlers without
+ * disturbing other processes.  (Other processes might share the signal
+ * table via the CLONE_SIGHAND option to clone().)
+ */
 static int unshare_sighand(struct task_struct *me)
 {
 	struct sighand_struct *oldsighand = me->sighand;
@@ -1296,13 +1297,18 @@ void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
  * Calling this is the point of no return. None of the failures will be
  * seen by userspace since either the process is already taking a fatal
  * signal (via de_thread() or coredump), or will have SEGV raised
- * (after exec_mmap()) by search_binary_handlers (see below).
+ * (after exec_mmap()) by search_binary_handler (see below).
  */
 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.
 	 */
@@ -1325,13 +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
-	 * search_binary_handler() will SEGV current.
-	 */
-	bprm->point_of_no_return = true;
 	bprm->mm = NULL;
 
 #ifdef CONFIG_POSIX_TIMERS
@@ -1720,13 +1719,8 @@ int search_binary_handler(struct linux_binprm *bprm)
 
 		read_lock(&binfmt_lock);
 		put_binfmt(fmt);
-		if (retval < 0 && bprm->point_of_no_return) {
-			/* we got to flush_old_exec() and failed after it */
-			read_unlock(&binfmt_lock);
-			force_sigsegv(SIGSEGV);
-			return retval;
-		}
-		if (retval != -ENOEXEC || !bprm->file) {
+		if (bprm->point_of_no_return || !bprm->file ||
+		    (retval != -ENOEXEC)) {
 			read_unlock(&binfmt_lock);
 			return retval;
 		}
@@ -1897,6 +1891,14 @@ static int __do_execve_file(int fd, struct filename *filename,
 	return retval;
 
 out:
+	/*
+	 * If past the point of no return ensure the the code never
+	 * returns to the userspace process.  Use an existing fatal
+	 * signal if present otherwise terminate the process with
+	 * SIGSEGV.
+	 */
+	if (bprm->point_of_no_return && !fatal_signal_pending(current))
+		force_sigsegv(SIGSEGV);
 	if (bprm->mm) {
 		acct_arg_size(bprm, 0);
 		mmput(bprm->mm);