Fix a race between kern_setitimer() and realitexpire(), where the

callout is started before kern_setitimer() acquires process mutex, but
looses a race and kern_setitimer() gets the process mutex before the
callout.  Then, assuming that new specified struct itimerval has
it_interval zero, but it_value non-zero, the callout, after it starts
executing again, clears p->p_realtimer.it_value, but kern_setitimer()
already rescheduled the callout.

As the result of the race, both p_realtimer is zero, and the callout
is rescheduled. Then, in the exit1(), the exit code sees that it_value
is zero and does not even try to stop the callout. This allows the
struct proc to be reused and eventually the armed callout is
re-initialized.  The consequence is the corrupted callwheel tailq.

Use process mutex to interlock the callout start, which fixes the race.

Reported and tested by:	pho
Reviewed by:	jhb
MFC after:	2 weeks

3 files changed, 2 insertions, 5 deletions

diff --git a/sys/kern/init_main.c b/sys/kern/init_main.c
index c30e1a2..00b1c3f 100644
--- a/sys/kern/init_main.c
+++ b/sys/kern/init_main.c
@@ -498,7 +498,7 @@ proc0_init(void *dummy __unused)
 	strncpy(p->p_comm, "kernel", sizeof (p->p_comm));
 	strncpy(td->td_name, "swapper", sizeof (td->td_name));
 
-	callout_init(&p->p_itcallout, CALLOUT_MPSAFE);
+	callout_init_mtx(&p->p_itcallout, &p->p_mtx, 0);
 	callout_init_mtx(&p->p_limco, &p->p_mtx, 0);
 	callout_init(&td->td_slpcallout, CALLOUT_MPSAFE);
 
diff --git a/sys/kern/kern_fork.c b/sys/kern/kern_fork.c
index 5dc43ca..b8a4825 100644
--- a/sys/kern/kern_fork.c
+++ b/sys/kern/kern_fork.c
@@ -591,7 +591,7 @@ do_fork(struct thread *td, int flags, struct proc *p2, struct thread *td2,
 	LIST_INIT(&p2->p_children);
 	LIST_INIT(&p2->p_orphans);
 
-	callout_init(&p2->p_itcallout, CALLOUT_MPSAFE);
+	callout_init_mtx(&p2->p_itcallout, &p2->p_mtx, 0);
 
 	/*
 	 * If PF_FORK is set, the child process inherits the
diff --git a/sys/kern/kern_time.c b/sys/kern/kern_time.c
index c0e7831..97c288d 100644
--- a/sys/kern/kern_time.c
+++ b/sys/kern/kern_time.c
@@ -788,13 +788,11 @@ realitexpire(void *arg)
 	struct timeval ctv, ntv;
 
 	p = (struct proc *)arg;
-	PROC_LOCK(p);
 	kern_psignal(p, SIGALRM);
 	if (!timevalisset(&p->p_realtimer.it_interval)) {
 		timevalclear(&p->p_realtimer.it_value);
 		if (p->p_flag & P_WEXIT)
 			wakeup(&p->p_itcallout);
-		PROC_UNLOCK(p);
 		return;
 	}
 	for (;;) {
@@ -806,7 +804,6 @@ realitexpire(void *arg)
 			timevalsub(&ntv, &ctv);
 			callout_reset(&p->p_itcallout, tvtohz(&ntv) - 1,
 			    realitexpire, p);
-			PROC_UNLOCK(p);
 			return;
 		}
 	}