ACK: [PATCH] posix-timer: Properly check sigevent->sigev_notify

[Kleber Souza]

On 08/03/18 23:25, Tyler Hicks wrote:
> From: Thomas Gleixner <tglx at linutronix.de>
> 
> timer_create() specifies via sigevent->sigev_notify the signal delivery for
> the new timer. The valid modes are SIGEV_NONE, SIGEV_SIGNAL, SIGEV_THREAD
> and (SIGEV_SIGNAL | SIGEV_THREAD_ID).
> 
> The sanity check in good_sigevent() is only checking the valid combination
> for the SIGEV_THREAD_ID bit, i.e. SIGEV_SIGNAL, but if SIGEV_THREAD_ID is
> not set it accepts any random value.
> 
> This has no real effects on the posix timer and signal delivery code, but
> it affects show_timer() which handles the output of /proc/$PID/timers. That
> function uses a string array to pretty print sigev_notify. The access to
> that array has no bound checks, so random sigev_notify cause access beyond
> the array bounds.
> 
> Add proper checks for the valid notify modes and remove the SIGEV_THREAD_ID
> masking from various code pathes as SIGEV_NONE can never be set in
> combination with SIGEV_THREAD_ID.
> 
> Reported-by: Eric Biggers <ebiggers3 at gmail.com>
> Reported-by: Dmitry Vyukov <dvyukov at google.com>
> Reported-by: Alexey Dobriyan <adobriyan at gmail.com>
> Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
> Cc: John Stultz <john.stultz at linaro.org>
> Cc: stable at vger.kernel.org
> 
> CVE-2017-18344
> 
> (backported from commit cef31d9af908243421258f1df35a4a644604efbe)
> [tyhicks: Do not worry about removing the SIGEV_THREAD_ID masking since it is
>  irrelevant to the security fix]
> Signed-off-by: Tyler Hicks <tyhicks at canonical.com>

Acked-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>

> ---
>  kernel/posix-timers.c | 25 +++++++++++++++----------
>  1 file changed, 15 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
> index 77e6b83c0431..26b910462a0c 100644
> --- a/kernel/posix-timers.c
> +++ b/kernel/posix-timers.c
> @@ -498,17 +498,22 @@ static struct pid *good_sigevent(sigevent_t * event)
>  {
>  	struct task_struct *rtn = current->group_leader;
>  
> -	if ((event->sigev_notify & SIGEV_THREAD_ID ) &&
> -		(!(rtn = find_task_by_vpid(event->sigev_notify_thread_id)) ||
> -		 !same_thread_group(rtn, current) ||
> -		 (event->sigev_notify & ~SIGEV_THREAD_ID) != SIGEV_SIGNAL))
> +	switch (event->sigev_notify) {
> +	case SIGEV_SIGNAL | SIGEV_THREAD_ID:
> +		rtn = find_task_by_vpid(event->sigev_notify_thread_id);
> +		if (!rtn || !same_thread_group(rtn, current))
> +			return NULL;
> +		/* FALLTHRU */
> +	case SIGEV_SIGNAL:
> +	case SIGEV_THREAD:
> +		if (event->sigev_signo <= 0 || event->sigev_signo > SIGRTMAX)
> +			return NULL;
> +		/* FALLTHRU */
> +	case SIGEV_NONE:
> +		return task_pid(rtn);
> +	default:
>  		return NULL;
> -
> -	if (((event->sigev_notify & ~SIGEV_THREAD_ID) != SIGEV_NONE) &&
> -	    ((event->sigev_signo <= 0) || (event->sigev_signo > SIGRTMAX)))
> -		return NULL;
> -
> -	return task_pid(rtn);
> +	}
>  }
>  
>  void posix_timers_register_clock(const clockid_t clock_id,
>