Change msleep() and tsleep() to not alter the calling thread's priority

if the specified priority is zero.  This avoids a race where the calling
thread could read a snapshot of it's current priority, then a different
thread could change the first thread's priority, then the original thread
would call sched_prio() inside msleep() undoing the change made by the
second thread.  I used a priority of zero as no thread that calls msleep()
or tsleep() should be specifying a priority of zero anyway.

The various places that passed 'curthread->td_priority' or some variant
as the priority now pass 0.

1 files changed, 5 insertions, 3 deletions

diff --git a/sys/kern/kern_synch.c b/sys/kern/kern_synch.c
index d2c6acc..d84c64f 100644
--- a/sys/kern/kern_synch.c
+++ b/sys/kern/kern_synch.c
@@ -195,9 +195,11 @@ msleep(ident, mtx, priority, wmesg, timo)
 	/*
 	 * Adjust this thread's priority.
 	 */
-	mtx_lock_spin(&sched_lock);
-	sched_prio(td, priority & PRIMASK);
-	mtx_unlock_spin(&sched_lock);
+	if ((priority & PRIMASK) != 0) {
+		mtx_lock_spin(&sched_lock);
+		sched_prio(td, priority & PRIMASK);
+		mtx_unlock_spin(&sched_lock);
+	}
 
 	if (timo && catch)
 		rval = sleepq_timedwait_sig(ident);