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

[Kees Cook]

On Mon, Oct 25, 2021 at 08:33:05AM +0000, Yafang Shao wrote:
> 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@xxxxxxxxxxxx>
> Suggested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx>
> Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Petr Mladek <pmladek@xxxxxxxx>
> ---
>  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 */

This may could say "always NUL terminated and zero-padded"

> +	strscpy_pad(buf, tsk->comm, buf_size);
>  	task_unlock(tsk);
>  	return buf;
>  }
> -- 
> 2.17.1
> 

But for the replacement with strscpy_pad(), yes please:

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>


-- 
Kees Cook