[PATCH v2] sched/eevdf: Prevent vlag from going out of bounds when reweight_eevdf

[Xuewen Yan]

kernel encounters the following error when running workload:

BUG: kernel NULL pointer dereference, address: 0000002c
EIP: set_next_entity (fair.c:?)

which was caused by NULL pointer returned by pick_eevdf().

Further investigation has shown that, the entity_eligible() has an
false-negative issue when the entity's vruntime is far behind the
cfs_rq.min_vruntime that, the (vruntime - cfs_rq->min_vruntime) * load
caused a s64 overflow, thus every entity on the rb-tree is not
eligible, which results in a NULL candidate.

The reason why entity's vruntime is far behind the cfs_rq.min_vruntime
is because during a on_rq task group's update_cfs_group()->reweight_eevdf(),
there is no limit on the new entity's vlag. If the new weight is much
smaller than the old one,

vlag = div_s64(vlag * old_weight, weight)

generates a huge vlag, and results in very small(negative) vruntime.

Thus limit the range of vlag accordingly.

Reported-by: Sergei Trofimovich <slyich@xxxxxxxxx>
Closes: https://lore.kernel.org/all/ZhuYyrh3mweP_Kd8@xxxxxxx/
Reported-by: Igor Raits <igor@xxxxxxxxxxxx>
Closes: https://lore.kernel.org/all/CA+9S74ih+45M_2TPUY_mPPVDhNvyYfy1J1ftSix+KjiTVxg8nw@xxxxxxxxxxxxxx/
Reported-by: Breno Leitao <leitao@xxxxxxxxxx>
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/lkml/202401301012.2ed95df0-oliver.sang@xxxxxxxxx/
Reported-by: Yujie Liu <yujie.liu@xxxxxxxxx>
Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
---
changes of v2:
-add reported-by (suggested by <yu.c.chen@xxxxxxxxx>)
-remork the changelog (<yu.c.chen@xxxxxxxxx>)
-remove the judgement of fork (Peter)
-remove the !on_rq case. (Peter)
---
Previous discussion link:
https://lore.kernel.org/lkml/20240226082349.302363-1-yu.c.chen@xxxxxxxxx/
https://lore.kernel.org/all/20240130080643.1828-1-xuewen.yan@xxxxxxxxxx/
---
---
 kernel/sched/fair.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 03be0d1330a6..64826f406d6d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -696,15 +696,21 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
  *
  * XXX could add max_slice to the augmented data to track this.
  */
-static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
+static s64 entity_lag(u64 avruntime, struct sched_entity *se)
 {
-	s64 lag, limit;
+	s64 vlag, limit;
+
+	vlag = avruntime - se->vruntime;
+	limit = calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se);
+
+	return clamp(vlag, -limit, limit);
+}
 
+static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
+{
 	SCHED_WARN_ON(!se->on_rq);
-	lag = avg_vruntime(cfs_rq) - se->vruntime;
 
-	limit = calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se);
-	se->vlag = clamp(lag, -limit, limit);
+	se->vlag = entity_lag(avg_vruntime(cfs_rq), se);
 }
 
 /*
@@ -3761,7 +3767,7 @@ static void reweight_eevdf(struct cfs_rq *cfs_rq, struct sched_entity *se,
 	 *	   = V  - vl'
 	 */
 	if (avruntime != se->vruntime) {
-		vlag = (s64)(avruntime - se->vruntime);
+		vlag = entity_lag(avruntime, se);
 		vlag = div_s64(vlag * old_weight, weight);
 		se->vruntime = avruntime - vlag;
 	}
-- 
2.25.1