From 11f9ca696ee12e1732dd2f15dd8660e0689194b6 Mon Sep 17 00:00:00 2001
From: Mark Johnston <markj@FreeBSD.org>
Date: Sat, 9 Jan 2016 01:56:46 +0000
Subject: [PATCH] Prevent cv_waiters wraparound.

r282971 attempted to fix this problem by decrementing cv_waiters after
waking up from sleeping on a condition variable, but this can result in
a use-after-free if the CV is freed before all woken threads have had a
chance to run. Instead, avoid incrementing cv_waiters past INT_MAX, and
have cv_signal() explicitly check for sleeping threads once cv_waiters has
reached this bound.

Reviewed by:	jhb
MFC after:	2 weeks
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D4822
---
 sys/kern/kern_condvar.c | 33 ++++++++++++++++++++++++++-------
 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/sys/kern/kern_condvar.c b/sys/kern/kern_condvar.c
index 2700a25d477c..95a6d09e1775 100644
--- a/sys/kern/kern_condvar.c
+++ b/sys/kern/kern_condvar.c
@@ -31,6 +31,7 @@ __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
 #include <sys/systm.h>
+#include <sys/limits.h>
 #include <sys/lock.h>
 #include <sys/mutex.h>
 #include <sys/proc.h>
@@ -46,6 +47,17 @@ __FBSDID("$FreeBSD$");
 #include <sys/ktrace.h>
 #endif
 
+/*
+ * A bound below which cv_waiters is valid.  Once cv_waiters reaches this bound,
+ * cv_signal must manually check the wait queue for threads.
+ */
+#define	CV_WAITERS_BOUND	INT_MAX
+
+#define	CV_WAITERS_INC(cvp) do {					\
+	if ((cvp)->cv_waiters < CV_WAITERS_BOUND)			\
+		(cvp)->cv_waiters++;					\
+} while (0)
+
 /*
  * Common sanity checks for cv_wait* functions.
  */
@@ -122,7 +134,7 @@ _cv_wait(struct cv *cvp, struct lock_object *lock)
 
 	sleepq_lock(cvp);
 
-	cvp->cv_waiters++;
+	CV_WAITERS_INC(cvp);
 	if (lock == &Giant.lock_object)
 		mtx_assert(&Giant, MA_OWNED);
 	DROP_GIANT();
@@ -184,7 +196,7 @@ _cv_wait_unlock(struct cv *cvp, struct lock_object *lock)
 
 	sleepq_lock(cvp);
 
-	cvp->cv_waiters++;
+	CV_WAITERS_INC(cvp);
 	DROP_GIANT();
 
 	sleepq_add(cvp, lock, cvp->cv_description, SLEEPQ_CONDVAR, 0);
@@ -240,7 +252,7 @@ _cv_wait_sig(struct cv *cvp, struct lock_object *lock)
 
 	sleepq_lock(cvp);
 
-	cvp->cv_waiters++;
+	CV_WAITERS_INC(cvp);
 	if (lock == &Giant.lock_object)
 		mtx_assert(&Giant, MA_OWNED);
 	DROP_GIANT();
@@ -307,7 +319,7 @@ _cv_timedwait_sbt(struct cv *cvp, struct lock_object *lock, sbintime_t sbt,
 
 	sleepq_lock(cvp);
 
-	cvp->cv_waiters++;
+	CV_WAITERS_INC(cvp);
 	if (lock == &Giant.lock_object)
 		mtx_assert(&Giant, MA_OWNED);
 	DROP_GIANT();
@@ -376,7 +388,7 @@ _cv_timedwait_sig_sbt(struct cv *cvp, struct lock_object *lock,
 
 	sleepq_lock(cvp);
 
-	cvp->cv_waiters++;
+	CV_WAITERS_INC(cvp);
 	if (lock == &Giant.lock_object)
 		mtx_assert(&Giant, MA_OWNED);
 	DROP_GIANT();
@@ -422,8 +434,15 @@ cv_signal(struct cv *cvp)
 	wakeup_swapper = 0;
 	sleepq_lock(cvp);
 	if (cvp->cv_waiters > 0) {
-		cvp->cv_waiters--;
-		wakeup_swapper = sleepq_signal(cvp, SLEEPQ_CONDVAR, 0, 0);
+		if (cvp->cv_waiters == CV_WAITERS_BOUND &&
+		    sleepq_lookup(cvp) == NULL) {
+			cvp->cv_waiters = 0;
+		} else {
+			if (cvp->cv_waiters < CV_WAITERS_BOUND)
+				cvp->cv_waiters--;
+			wakeup_swapper = sleepq_signal(cvp, SLEEPQ_CONDVAR, 0,
+			    0);
+		}
 	}
 	sleepq_release(cvp);
 	if (wakeup_swapper)