Re: [PATCH v2] kthread, tracing: Don't expose half-written comm when creating kthreads

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 24 Jul 2018 17:12:13 +0200
Snild Dolkow <snild@sony.com> wrote:

> There is a window for racing when printing directly to task->comm,
> allowing other threads to see a non-terminated string. The vsnprintf
> function fills the buffer, counts the truncated chars, then finally
> writes the \0 at the end.
> 
> 	creator                     other
> 	vsnprintf:
> 	  fill (not terminated)
> 	  count the rest            trace_sched_waking(p):
> 	  ...                         memcpy(comm, p->comm, TASK_COMM_LEN)
> 	  write \0
> 
> The consequences depend on how 'other' uses the string. In our case,
> it was copied into the tracing system's saved cmdlines, a buffer of
> adjacent TASK_COMM_LEN-byte buffers (note the 'n' where 0 should be):
> 
> 	crash-arm64> x/1024s savedcmd->saved_cmdlines | grep 'evenk'
> 	0xffffffd5b3818640:     "irq/497-pwr_evenkworker/u16:12"
> 
> ...and a strcpy out of there would cause stack corruption:
> 
> 	[224761.522292] Kernel panic - not syncing: stack-protector:
> 	    Kernel stack is corrupted in: ffffff9bf9783c78
> 
> 	crash-arm64> kbt | grep 'comm\|trace_print_context'
> 	#6  0xffffff9bf9783c78 in trace_print_context+0x18c(+396)
> 	      comm (char [16]) =  "irq/497-pwr_even"
> 
> 	crash-arm64> rd 0xffffffd4d0e17d14 8
> 	ffffffd4d0e17d14:  2f71726900000000 5f7277702d373934   ....irq/497-pwr_
> 	ffffffd4d0e17d24:  726f776b6e657665 3a3631752f72656b   evenkworker/u16:
> 	ffffffd4d0e17d34:  f9780248ff003231 cede60e0ffffff9b   12..H.x......`..
> 	ffffffd4d0e17d44:  cede60c8ffffffd4 00000fffffffffd4   .....`..........
> 
> The workaround in e09e28671 (use strlcpy in __trace_find_cmdline) was
> likely needed because of this same bug.
> 
> Solved by vsnprintf:ing to a local buffer, then using set_task_comm().
> This way, there won't be a window where comm is not terminated.
> 

Should add:

Cc: stable@vger.kernel.org

And it is a bug from beginning of git. But it wasn't really until
tracing came along that can trigger it. Thus:

Fixes: bc0c38d139ec7 ("ftrace: latency tracer infrastructure")

> Signed-off-by: Snild Dolkow <snild@sony.com>

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Now the question is, who will take this in their tree?

I can, unless someone else wants it. But I wont without another
Acked-by.

-- Steve

> ---
>  kernel/kthread.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 481951bf091d..1a481ae12dec 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -319,8 +319,14 @@ 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);
> +		/*
> +		 * task is already visible to other tasks, so updating
> +		 * COMM must be protected.
> +		 */
> +		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.