Re: [PATCH v2] exec: avoid gcc-8 warning for get_task_comm

[Kees Cook]

On Tue, Dec 5, 2017 at 7:17 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 keeps the strncpy() to ensure we have properly padded target buffer,
> but ensures that we use the correct length, by passing the actual length
> of the destination buffer as well as adding a build-time check to ensure
> it is exactly TASK_COMM_LEN.  There are only 23 callsights which I all
> reviewed to ensure this is currently the case. We could get away with
> doing only the check or passing the right length, but it doesn't hurt
> to do both.
>
> Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Awesome. Thanks for checking them all!

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  fs/exec.c             | 7 +++----
>  include/linux/sched.h | 6 +++++-
>  2 files changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 6be2aa0ab26f..156f56acfe8e 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1216,15 +1216,14 @@ static int de_thread(struct task_struct *tsk)
>         return -EAGAIN;
>  }
>
> -char *get_task_comm(char *buf, struct task_struct *tsk)
> +char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
>  {
> -       /* buf must be at least sizeof(tsk->comm) in size */
>         task_lock(tsk);
> -       strncpy(buf, tsk->comm, sizeof(tsk->comm));
> +       strncpy(buf, tsk->comm, buf_size);
>         task_unlock(tsk);
>         return buf;
>  }
> -EXPORT_SYMBOL_GPL(get_task_comm);
> +EXPORT_SYMBOL_GPL(__get_task_comm);
>
>  /*
>   * These functions flushes out all traces of the currently running executable
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 21991d668d35..5124ba709830 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1503,7 +1503,11 @@ static inline void set_task_comm(struct task_struct *tsk, const char *from)
>         __set_task_comm(tsk, from, false);
>  }
>
> -extern char *get_task_comm(char *to, struct task_struct *tsk);
> +extern char *__get_task_comm(char *to, size_t len, struct task_struct *tsk);
> +#define get_task_comm(buf, tsk) ({                     \
> +       BUILD_BUG_ON(sizeof(buf) != TASK_COMM_LEN);     \
> +       __get_task_comm(buf, sizeof(buf), tsk);         \
> +})
>
>  #ifdef CONFIG_SMP
>  void scheduler_ipi(void);
> --
> 2.9.0
>



-- 
Kees Cook
Pixel Security