[tip: sched/core] sched/fair: Fix overflow in update_tg_cfs_runnable()

[tip-bot2 for Chen, Yu C]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     4f166adb5cb0525d9e32d45729fd8f28c80acbee
Gitweb:        https://git.kernel.org/tip/4f166adb5cb0525d9e32d45729fd8f28c80acbee
Author:        Chen, Yu C <yu.c.chen@xxxxxxxxx>
AuthorDate:    Sat, 20 Jun 2026 11:54:22 +08:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 30 Jun 2026 10:56:51 +02:00

sched/fair: Fix overflow in update_tg_cfs_runnable()

A divide-by-zero crash is observed when running hackbench:

   [14697.488452] CPU: 112 UID: 0 PID: 124791 Comm: hackbench Not tainted 7.1.0-rc2+
   [14697.492627] RIP: 0010:propagate_entity_load_avg+0x35f/0x3e0
   [14697.506799]  <TASK>
   [14697.507411]  __dequeue_task+0x2b4/0xc70
   [14697.508677]  dequeue_task_fair+0x36/0x370
   [14697.509047]  dequeue_task+0x101/0x2f0
   [14697.509426]  __schedule+0x1b1/0x1a00
   [14697.510868]  anon_pipe_read+0x3da/0x450
   [14697.511400]  vfs_read+0x361/0x390
   [14697.512053]  __x64_sys_read+0x19/0x30

The divide-by-zero happens here:

  if (scale_load_down(gcfs_rq->load.weight)) {
           load_sum = div_u64(gcfs_rq->avg.load_sum,
                              scale_load_down(gcfs_rq->load.weight));
  }

gcfs_rq->load.weight is an insane large value and is truncated
to the lower 32 bits by div_u64, which happen to be 0.

Using AI for investigation, the cause is a u32 overflow in
update_tg_cfs_runnable(), and flat pickup became a victim when using
tg_tasks():

   u32 new_sum, divider;
   ...
   new_sum = se->avg.runnable_avg * divider; <-- boom

The following sequence shows how this triggers the crash:

   propagate_entity_load_avg()
     update_tg_cfs_runnable()     # u32 overflow corrupts runnable_sum

   __update_load_avg_cfs_rq()
     ___update_load_avg()         # computes insane runnable_avg
   update_tg_load_avg()           # propagates to tg->runnable_avg

   update_cfs_group()
     calc_concur_shares()
       tg_tasks()                 # long-to-int truncation, negative nr
     reweight_entity()            # corrupted se->load.weight
       update_load_add()          # corrupted cfs_rq->load.weight

   propagate_entity_load_avg()
     update_tg_cfs_load()
       div_u64()                  # divide-by-zero

Fix by widening new_sum from u32 to u64 (no need to force tg_tasks()
to return unsigned long after this fix)

Fixes: 95246d1ec80b ("sched/pelt: Relax the sync of runnable_sum with runnable_avg")
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/a22eea2b-4c4a-4623-9a44-d7b18c0c91c8@xxxxxxxxx
---
 kernel/sched/fair.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a384f16..e89edbd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5174,7 +5174,8 @@ static inline void
 update_tg_cfs_runnable(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq *gcfs_rq)
 {
 	long delta_sum, delta_avg = gcfs_rq->avg.runnable_avg - se->avg.runnable_avg;
-	u32 new_sum, divider;
+	u64 new_sum;
+	u32 divider;
 
 	/* Nothing to update */
 	if (!delta_avg)
@@ -5188,7 +5189,7 @@ update_tg_cfs_runnable(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cf
 
 	/* Set new sched_entity's runnable */
 	se->avg.runnable_avg = gcfs_rq->avg.runnable_avg;
-	new_sum = se->avg.runnable_avg * divider;
+	new_sum = (u64)se->avg.runnable_avg * divider;
 	delta_sum = (long)new_sum - (long)se->avg.runnable_sum;
 	se->avg.runnable_sum = new_sum;