Re: [PATCH 3.16 298/366] kthread, tracing: Don't expose half-written comm when creating kthreads

[Snild Dolkow]

On 11/11/18 8:49 PM, Ben Hutchings wrote:
> 3.16.61-rc1 review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Snild Dolkow <snild@xxxxxxxx>
> 
> commit 3e536e222f2930534c252c1cc7ae799c725c5ff9 upstream.
> 
> 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.
> 
> Link: http://lkml.kernel.org/r/20180726071539.188015-1-snild@xxxxxxxx
> 
> Fixes: bc0c38d139ec7 ("ftrace: latency tracer infrastructure")
> Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> Signed-off-by: Snild Dolkow <snild@xxxxxxxx>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> [bwh: Backported to 3.16: adjust context]
> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> ---
>  kernel/kthread.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -309,10 +309,16 @@ struct task_struct *kthread_create_on_no
>  	if (!IS_ERR(task)) {
>  		static const struct sched_param param = { .sched_priority = 0 };
>  		va_list args;
> +		char name[TASK_COMM_LEN];
>  
> +		/*
> +		 * task is already visible to other tasks, so updating
> +		 * COMM must be protected.
> +		 */
>  		va_start(args, namefmt);
> -		vsnprintf(task->comm, sizeof(task->comm), namefmt, args);
> +		vsnprintf(name, sizeof(name), namefmt, args);
>  		va_end(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.
> 

Reviewed-by: Snild Dolkow <snild@xxxxxxxx>