Re: UBSAN: shift-out-of-bounds in load_balance

[Valentin Schneider]

+Vincent

On 22/02/21 09:12, syzbot wrote:
> syzbot has found a reproducer for the following issue on:
>
> HEAD commit:    31caf8b2 Merge branch 'linus' of git://git.kernel.org/pub/..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=16ab2682d00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=b81388f0b32761d4
> dashboard link: https://syzkaller.appspot.com/bug?extid=d7581744d5fd27c9fbe1
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1277457f500000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+d7581744d5fd27c9fbe1@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> ================================================================================
> UBSAN: shift-out-of-bounds in kernel/sched/fair.c:7712:14
> shift exponent 149 is too large for 64-bit type 'long unsigned int'

That 149 is surprising.

sd->cache_nice_tries is \in {1, 2}, and sd->nr_balanced_failed should be in
the same ballpark.

A successful load_balance() resets it to 0; a failed one increments
it. Once it gets to sd->cache_nice_tries + 3, this should trigger an active
balance, which will either set it to sd->cache_nice_tries+1 or reset it to
0. There is this one condition that could let it creep up uncontrollably:

  /*
   * Don't kick the active_load_balance_cpu_stop,
   * if the curr task on busiest CPU can't be
   * moved to this_cpu:
   */
  if (!cpumask_test_cpu(this_cpu, busiest->curr->cpus_ptr)) {
          raw_spin_unlock_irqrestore(&busiest->lock,
                                      flags);
          goto out_one_pinned;
  }

So despite the resulting sd->balance_interval increase, repeatedly hitting
this might yield the above. Would we then want something like this?

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8a8bd7b13634..b65c24b5ae91 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7422,6 +7422,11 @@ struct lb_env {
 	struct list_head	tasks;
 };
 
+static inline unsigned int sd_balance_failed_cap(struct sched_domain *sd)
+{
+	return sd->cache_nice_tries + 3;
+}
+
 /*
  * Is this task likely cache-hot:
  */
@@ -9493,7 +9498,7 @@ imbalanced_active_balance(struct lb_env *env)
 	 * threads on a system with spare capacity
 	 */
 	if ((env->migration_type == migrate_task) &&
-	    (sd->nr_balance_failed > sd->cache_nice_tries+2))
+	    (sd->nr_balance_failed >= sd_balance_failed_cap(sd)))
 		return 1;
 
 	return 0;
@@ -9737,8 +9742,10 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 		 * frequent, pollute the failure counter causing
 		 * excessive cache_hot migrations and active balances.
 		 */
-		if (idle != CPU_NEWLY_IDLE)
-			sd->nr_balance_failed++;
+		if (idle != CPU_NEWLY_IDLE) {
+			sd->nr_balance_failed = min(sd->nr_balance_failed + 1,
+						    sd_balance_failed_cap(sd));
+		}
 
 		if (need_active_balance(&env)) {
 			unsigned long flags;