| Date | Fri, 5 May 2023 16:50:20 +0200 | From | Frederic Weisbecker <> | Subject | Re: [patch 02/20] posix-timers: Ensure timer ID search-loop limit is valid |
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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@syzkaller.appspotmail.com > Reported-by: Dmitry Vyukov <dvyukov@google.com> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de> > --- > 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.
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