linux-kernel - Re: [patch v2 02/20] posix-timers: Ensure timer ID search-loop limit is valid

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Date:   Mon, 5 Jun 2023 16:17:04 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        syzbot+5c54bd3eb218bb595aa9@...kaller.appspotmail.com,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [patch v2 02/20] posix-timers: Ensure timer ID search-loop limit
 is valid

Le Thu, Jun 01, 2023 at 08:58:47PM +0200, Thomas Gleixner a écrit :
> 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 all id operations are under the hash lock.
> 
> Reported-by: syzbot+5c54bd3eb218bb595aa9@...kaller.appspotmail.com
> Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>