RE: [PATCH] exec: Make sure task->comm is always NUL-terminated

[David Laight <David.Laight@ACULAB.COM>]

From: Kees Cook
> Sent: 30 November 2024 04:49
>
> Instead of adding a new use of the ambiguous strncpy(), we'd want to
> use memtostr_pad() which enforces being able to check at compile time
> that sizes are sensible, but this requires being able to see string
> buffer lengths. Instead of trying to inline __set_task_comm() (which
> needs to call trace and perf functions), just open-code it. But to
> make sure we're always safe, add compile-time checking like we already
> do for get_task_comm().
...
> Here's what I'd prefer to use to clean up set_task_comm(). I merged
> Linus and Eric's suggestions and open-coded memtostr_pad().
> ---
>  fs/exec.c             | 12 ++++++------
>  include/linux/sched.h |  9 ++++-----
>  io_uring/io-wq.c      |  2 +-
>  io_uring/sqpoll.c     |  2 +-
>  kernel/kthread.c      |  3 ++-
>  5 files changed, 14 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index e0435b31a811..5f16500ac325 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1200,16 +1200,16 @@ char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
>  EXPORT_SYMBOL_GPL(__get_task_comm);
> 
>  /*
> - * These functions flushes out all traces of the currently running executable
> - * so that a new one can be started
> + * This is unlocked -- the string will always be NUL-terminated, but
> + * may show overlapping contents if racing concurrent reads.
>   */
> -
>  void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
>  {
> -	task_lock(tsk);
> +	size_t len = min(strlen(buf), sizeof(tsk->comm) - 1);
> +
>  	trace_task_rename(tsk, buf);
> -	strscpy_pad(tsk->comm, buf, sizeof(tsk->comm));
> -	task_unlock(tsk);
> +	memcpy(tsk->comm, buf, len);
> +	memset(&tsk->comm[len], 0, sizeof(tsk->comm) - len);
>  	perf_event_comm(tsk, exec);

Why not do strscpy_pad() into a local char[16] and then do a 16 byte
memcpy() into the target buffer?

Then non-constant input data will always give a valid '\0' terminated string
regardless of how strscpy_pad() is implemented.

	David

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