git: 347ae5f3457b - stable/13 - kqueue timer: Remove detached knotes from the process stop queue

[Mark Johnston]

The branch stable/13 has been updated by markj:

URL: https://cgit.FreeBSD.org/src/commit/?id=347ae5f3457bfe316aa2fb3c27dd7f431da6e45a

commit 347ae5f3457bfe316aa2fb3c27dd7f431da6e45a
Author:     Mark Johnston <markj at FreeBSD.org>
AuthorDate: 2021-05-14 14:07:56 +0000
Commit:     Mark Johnston <markj at FreeBSD.org>
CommitDate: 2021-05-21 12:45:34 +0000

    kqueue timer: Remove detached knotes from the process stop queue
    
    There are some scenarios where a timer event may be detached when it is
    on the process' kqueue timer stop queue.  If kqtimer_proc_continue() is
    called after that point, it will iterate over the queue and access freed
    timer structures.
    
    It is also possible, at least in a multithreaded program, for a stopped
    timer event to be scheduled without removing it from the process' stop
    queue.  Ensure that we do not doubly enqueue the event structure in this
    case.
    
    Reported by:    syzbot+cea0931bb4e34cd728bd at syzkaller.appspotmail.com
    Reported by:    syzbot+9e1a2f3734652015998c at syzkaller.appspotmail.com
    Reviewed by:    kib
    Sponsored by:   The FreeBSD Foundation
    Differential Revision:  https://reviews.freebsd.org/D30251
    
    (cherry picked from commit 2cca77ee01343bf080f1b70f0217a84c200fe7c1)
---
 sys/kern/kern_event.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c
index 03f4b3afbc28..6b8ab11cb2dc 100644
--- a/sys/kern/kern_event.c
+++ b/sys/kern/kern_event.c
@@ -680,11 +680,14 @@ struct kq_timer_cb_data {
 	struct proc *p;
 	struct knote *kn;
 	int cpuid;
+	int flags;
 	TAILQ_ENTRY(kq_timer_cb_data) link;
 	sbintime_t next;	/* next timer event fires at */
 	sbintime_t to;		/* precalculated timer period, 0 for abs */
 };
 
+#define	KQ_TIMER_CB_ENQUEUED	0x01
+
 static void
 kqtimer_sched_callout(struct kq_timer_cb_data *kc)
 {
@@ -706,6 +709,7 @@ kqtimer_proc_continue(struct proc *p)
 
 	TAILQ_FOREACH_SAFE(kc, &p->p_kqtim_stop, link, kc1) {
 		TAILQ_REMOVE(&p->p_kqtim_stop, kc, link);
+		kc->flags &= ~KQ_TIMER_CB_ENQUEUED;
 		if (kc->next <= now)
 			filt_timerexpire_l(kc->kn, true);
 		else
@@ -753,7 +757,10 @@ filt_timerexpire_l(struct knote *kn, bool proc_locked)
 		if (!proc_locked)
 			PROC_LOCK(p);
 		if (P_SHOULDSTOP(p) || P_KILLED(p)) {
-			TAILQ_INSERT_TAIL(&p->p_kqtim_stop, kc, link);
+			if ((kc->flags & KQ_TIMER_CB_ENQUEUED) == 0) {
+				kc->flags |= KQ_TIMER_CB_ENQUEUED;
+				TAILQ_INSERT_TAIL(&p->p_kqtim_stop, kc, link);
+			}
 			if (!proc_locked)
 				PROC_UNLOCK(p);
 			return;
@@ -826,6 +833,7 @@ filt_timerattach(struct knote *kn)
 	kc->kn = kn;
 	kc->p = curproc;
 	kc->cpuid = PCPU_GET(cpuid);
+	kc->flags = 0;
 	callout_init(&kc->c, 1);
 	filt_timerstart(kn, to);
 
@@ -856,6 +864,11 @@ filt_timerdetach(struct knote *kn)
 
 	kc = kn->kn_ptr.p_v;
 	callout_drain(&kc->c);
+	if ((kc->flags & KQ_TIMER_CB_ENQUEUED) != 0) {
+		PROC_LOCK(kc->p);
+		TAILQ_REMOVE(&kc->p->p_kqtim_stop, kc, link);
+		PROC_UNLOCK(kc->p);
+	}
 	free(kc, M_KQUEUE);
 	old = atomic_fetchadd_int(&kq_ncallouts, -1);
 	KASSERT(old > 0, ("Number of callouts cannot become negative"));