linux-kernel - Re: [PATCH RESEND] kthread, tracing: Don't expose half-written comm when creating kthreads

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Message-ID: <20180723095550.08203a24@gandalf.local.home>
Date:   Mon, 23 Jul 2018 09:55:50 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Snild Dolkow <snild@...y.com>
Cc:     <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Enderborg <peter.enderborg@...y.com>,
        Yoshitaka Seto <yoshitaka.seto@...y.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
        KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
        John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH RESEND] kthread, tracing: Don't expose half-written comm
 when creating kthreads

On Mon, 23 Jul 2018 15:42:10 +0200
Snild Dolkow <snild@...y.com> wrote:

> There was a window for racing when task->comm was being written. The
> vsnprintf function writes 16 bytes, then counts the rest, then null
> terminates. In the meantime, other threads could see the non-terminated
> comm value. In our case, it got into the trace system's saved cmdlines
> and could cause stack corruption when strcpy'd out of there.
> 
> The workaround in e09e28671 (use strlcpy in __trace_find_cmdline) was
> likely needed because of this bug.
> 
> Solved by vsnprintf:ing to a local buffer, then using set_task_comm().
> 
> Signed-off-by: Snild Dolkow <snild@...y.com>
> ---
>  kernel/kthread.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 481951bf091d..28874afbf747 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -319,8 +319,10 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
>  	task = create->result;
>  	if (!IS_ERR(task)) {
>  		static const struct sched_param param = { .sched_priority = 0 };
> +		char name[TASK_COMM_LEN];
>  
> -		vsnprintf(task->comm, sizeof(task->comm), namefmt, args);

Can you add a comment here stating something to the affect of:

		/* task is now visible to other tasks */

-- Steve

> +		vsnprintf(name, sizeof(name), namefmt, args);
> +		set_task_comm(task, name);
>  		/*
>  		 * root may have changed our (kthreadd's) priority or CPU mask.
>  		 * The kernel thread should not inherit these properties.