Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | Date | Wed, 23 Jul 2008 20:52:05 +0400 | From | Oleg Nesterov <> | Subject | [PATCH 1/2] posix-timers: fix posix_timer_event() vs dequeue_signal() race |
| |
The bug was reported and analysed by Mark McLoughlin <markmc@redhat.com>, the patch is based on his and Roland's suggestions.
posix_timer_event() always rewrites the pre-allocated siginfo before sending the signal. Most of the written info is the same all the time, but memset(0) is very wrong. If ->sigq is queued we can race with collect_signal() which can fail to find this siginfo looking at .si_signo, or copy_siginfo() can copy the wrong .si_code/si_tid/etc.
In short, sys_timer_settime() can in fact stop the active timer, or the user can receive the siginfo with the wrong .si_xxx values.
Move "memset(->info, 0)" from posix_timer_event() to alloc_posix_timer(), change send_sigqueue() to set .si_overrun = 0 when ->sigq is not queued. It would be nice to move the whole sigq->info initialization from send to create path, but this is not easy to do without uglifying timer_create() further.
As Roland rightly pointed out, we need more cleanups/fixes here, see the "FIXME" comment in the patch. Hopefully this patch makes sense anyway, and it can mask the most bad implications.
Reported-by: Mark McLoughlin <markmc@redhat.com> Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
posix-timers.c | 17 +++++++++++++---- signal.c | 1 + 2 files changed, 14 insertions(+), 4 deletions(-)
--- 26-rc2/kernel/posix-timers.c~1_PTE_QUEUED 2008-07-20 14:47:53.000000000 +0400 +++ 26-rc2/kernel/posix-timers.c 2008-07-23 15:04:18.000000000 +0400 @@ -296,14 +296,22 @@ void do_schedule_next_timer(struct sigin unlock_timer(timr, flags); } -int posix_timer_event(struct k_itimer *timr,int si_private) +int posix_timer_event(struct k_itimer *timr, int si_private) { - memset(&timr->sigq->info, 0, sizeof(siginfo_t)); + /* + * FIXME: if ->sigq is queued we can race with + * dequeue_signal()->do_schedule_next_timer(). + * + * If dequeue_signal() sees the "right" value of + * si_sys_private it calls do_schedule_next_timer(). + * We re-queue ->sigq and drop ->it_lock(). + * do_schedule_next_timer() locks the timer + * and re-schedules it while ->sigq is pending. + * Not really bad, but not that we want. + */ timr->sigq->info.si_sys_private = si_private; - /* Send signal to the process that owns this timer.*/ timr->sigq->info.si_signo = timr->it_sigev_signo; - timr->sigq->info.si_errno = 0; timr->sigq->info.si_code = SI_TIMER; timr->sigq->info.si_tid = timr->it_id; timr->sigq->info.si_value = timr->it_sigev_value; @@ -435,6 +443,7 @@ static struct k_itimer * alloc_posix_tim kmem_cache_free(posix_timers_cache, tmr); tmr = NULL; } + memset(&tmr->sigq->info, 0, sizeof(siginfo_t)); return tmr; } --- 26-rc2/kernel/signal.c~1_PTE_QUEUED 2008-07-06 19:29:27.000000000 +0400 +++ 26-rc2/kernel/signal.c 2008-07-23 13:55:11.000000000 +0400 @@ -1310,6 +1310,7 @@ int send_sigqueue(struct sigqueue *q, st q->info.si_overrun++; goto out; } + q->info.si_overrun = 0; signalfd_notify(t, sig); pending = group ? &t->signal->shared_pending : &t->pending;
| ![\](/images/icornerr.gif) |