[PATCH v6 02/12] fs/exec: make __get_task_comm always get a nul terminated string

[Yafang Shao <laoar.shao@gmail.com>]

If the dest buffer size is smaller than sizeof(tsk->comm), the buffer
will be without null ternimator, that may cause problem. We can make sure
the buffer size not smaller than comm at the callsite to avoid that
problem, but there may be callsite that we can't easily change.

Using strscpy_pad() instead of strncpy() in __get_task_comm() can make
the string always nul ternimated.

Suggested-by: Kees Cook <keescook@chromium.org>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kees Cook <keescook@chromium.org>
Cc: Petr Mladek <pmladek@suse.com>
---
 fs/exec.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index 404156b5b314..bf2a7a91eeea 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1209,7 +1209,8 @@ static int unshare_sighand(struct task_struct *me)
 char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
 {
 	task_lock(tsk);
-	strncpy(buf, tsk->comm, buf_size);
+	/* The copied value is always null terminated */
+	strscpy_pad(buf, tsk->comm, buf_size);
 	task_unlock(tsk);
 	return buf;
 }
-- 
2.17.1