Re: [PATCH] exec: avoid gcc-8 warning for get_task_comm

[Kees Cook]

On Mon, Dec 4, 2017 at 6:49 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> gcc-8 warns about using strncpy() with the source size as the limit:
>
> fs/exec.c:1223:32: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]
>
> This is indeed slightly suspicious, as it protects us from source
> arguments without NUL-termination, but does not guarantee that the
> destination is terminated.
>
> This changes it to strlcpy with a hardcoded length, to guarantee
> a properly terminated string. Since we already use strlcpy() for
> __set_task_comm(), the source should always be terminated properly,

It's terminated but it's not padded. This change _might_ result in
leaving a string buffer uninitialized (and if that buffer is passed as
a whole instead of just its strlen(), this could be a problem:
proc_comm_connector() e.g. may suffer from this, I haven't checked
closely).

> so this patch won't change the behavior, but make it a bit more robust.

I would rather we change get_task_comm() to include the size of buf...
maybe just convert it to a macro:

-char *get_task_comm(char *buf, struct task_struct *tsk)
+char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)

...

#define get_task_comm(buf, tsk) __get_task_comm(buf, sizeof(buf), tsk)

?

That will only work for preallocated arrays though. Maybe a build bug
when sizeof(buf) != TASK_COMM_LEN ?

I think the strncpy() should stay, but for other conversions where
NULL-padding isn't an issue, strscpy() is likely preferred to
strlcpy() to avoid the potential "overread" due to the strlen() call
in strlcpy().

-Kees

>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  fs/exec.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 6be2aa0ab26f..3e8012afe440 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1218,9 +1218,9 @@ static int de_thread(struct task_struct *tsk)
>
>  char *get_task_comm(char *buf, struct task_struct *tsk)
>  {
> -       /* buf must be at least sizeof(tsk->comm) in size */
> +       /* buf must be at least TASK_COMM_LEN in size */
>         task_lock(tsk);
> -       strncpy(buf, tsk->comm, sizeof(tsk->comm));
> +       strlcpy(buf, tsk->comm, TASK_COMM_LEN);
>         task_unlock(tsk);
>         return buf;
>  }
> --
> 2.9.0
>



-- 
Kees Cook
Pixel Security