Re: [patch 02/20] posix-timers: Ensure timer ID search-loop limit is valid

[Frederic Weisbecker]

On Tue, Apr 25, 2023 at 08:48:58PM +0200, Thomas Gleixner wrote:
> posix_timer_add() tries to allocate a posix timer ID by starting from the
> cached ID which was stored by the last successful allocation.
> 
> This is done in a loop searching the ID space for a free slot one by
> one. The loop has to terminate when the search wrapped around to the
> starting point.
> 
> But that's racy vs. establishing the starting point. That is read out
> lockless, which leads to the following problem:
> 
> CPU0	  	      	     	   CPU1
> posix_timer_add()
>   start = sig->posix_timer_id;
>   lock(hash_lock);
>   ...				   posix_timer_add()
>   if (++sig->posix_timer_id < 0)
>       			             start = sig->posix_timer_id;
>      sig->posix_timer_id = 0;
> 
> So CPU1 can observe a negative start value, i.e. -1, and the loop break
> never happens because the condition can never be true:
> 
>   if (sig->posix_timer_id == start)
>      break;
> 
> While this is unlikely to ever turn into an endless loop as the ID space is
> huge (INT_MAX), the racy read of the start value caught the attention of
> KCSAN and Dmitry unearthed that incorrectness.
> 
> Rewrite it so that the start condition can never observe the negative value
> and annotate the read and the write with READ_ONCE()/WRITE_ONCE().
> 
> Reported-by: syzbot+5c54bd3eb218bb595aa9@xxxxxxxxxxxxxxxxxxxxxxxxx
> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  include/linux/sched/signal.h |    2 +-
>  kernel/time/posix-timers.c   |   30 +++++++++++++++++-------------
>  2 files changed, 18 insertions(+), 14 deletions(-)
> 
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -135,7 +135,7 @@ struct signal_struct {
>  #ifdef CONFIG_POSIX_TIMERS
>  
>  	/* POSIX.1b Interval Timers */
> -	int			posix_timer_id;
> +	unsigned int		next_posix_timer_id;
>  	struct list_head	posix_timers;
>  
>  	/* ITIMER_REAL timer for the process */
> --- a/kernel/time/posix-timers.c
> +++ b/kernel/time/posix-timers.c
> @@ -140,25 +140,29 @@ static struct k_itimer *posix_timer_by_i
>  static int posix_timer_add(struct k_itimer *timer)
>  {
>  	struct signal_struct *sig = current->signal;
> -	int first_free_id = sig->posix_timer_id;
>  	struct hlist_head *head;
> -	int ret = -ENOENT;
> +	unsigned int start, id;
>  
> -	do {
> +	/* Can be written by a different task concurrently in the loop below */
> +	start = READ_ONCE(sig->next_posix_timer_id);
> +
> +	for (id = ~start; start != id; id++) {
>  		spin_lock(&hash_lock);
> -		head = &posix_timers_hashtable[hash(sig, sig->posix_timer_id)];
> -		if (!__posix_timers_find(head, sig, sig->posix_timer_id)) {
> +		id = sig->next_posix_timer_id;
> +
> +		/* Write the next ID back. Clamp it to the positive space */
> +		WRITE_ONCE(sig->next_posix_timer_id, (id + 1) & INT_MAX);

Isn't that looping forever?

Thanks.