[PATCH] sched/fair: Avoid overflow in place_entity()

[Hui Su]

place_entity() inflates an entity's virtual lag with

    lag * (load + weight) / load

Commit 4823725d9d1d ("sched/fair: Increase weight bits for avg_vruntime")
replaced the scaled-down weights used in this calculation with
avg_vruntime_weight(), removing the arithmetic headroom provided by
scale_load_down().

The multiplication can overflow s64 before the division even when the
final quotient is representable. A task's hierarchical effective weight
can be as small as 2 after avg_vruntime_weight() scaling under group
scheduling, while entity_lag() bounds |vlag| by

    calc_delta_fair(cfs_rq_max_slice(cfs_rq) + TICK_NSEC, se)

For example, with HZ=1000, NICE_0_LOAD=1048576, and the minimum scaled
hierarchical weight scale_load(2)=2048, this bound is 891289600. With 114
maximum-weight entities, the runqueue load is 10361604096. The
intermediate product is:

    891289600 * (10361604096 + 2)
        = 9235189971864780800
        > S64_MAX

The signed overflow corrupts the entity's placement.

Rewrite

    lag * (load + weight) / load

as

    lag + lag * weight / load

The two expressions are equivalent, but the latter avoids multiplying the
virtual lag by the total runqueue weight. entity_lag() bounds vlag against
the entity's effective weight, so the remaining lag * weight product stays
bounded. rescale_entity() preserves this scale across h_load changes and
already relies on the same product when rescaling vlag.

Keep the original zero-load warning semantics by multiplying lag by weight
after replacing the divisor with one.

Fixes: 4823725d9d1d ("sched/fair: Increase weight bits for avg_vruntime")
Signed-off-by: Hui Su <sh_def@xxxxxxx>
---
Testing:
- PASS: UBSAN positive/negative overflow cases, zero-load/control paths,
  64-bit landmarks NICE_0_LOAD=1048576 and scale_load(88761)=90891264,
  unscaled 32-bit landmarks 1024/88761, and 100000 random cases checked
  against a __int128 oracle.
- PASS: Full x86_64 kernel build; kernel/sched/fair.o builds for i386
  and arm64.

 kernel/sched/fair.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7455a83a6a99..a013bfba576b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6268,10 +6268,19 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 			load += avg_vruntime_weight(cfs_rq, curr->h_load.weight);
 
 		weight = avg_vruntime_weight(cfs_rq, se->h_load.weight);
-		lag *= load + weight;
-		if (WARN_ON_ONCE(!load))
+		if (WARN_ON_ONCE(!load)) {
 			load = 1;
-		lag = div64_long(lag, load);
+			lag *= weight;
+		} else {
+			/*
+			 * Avoid overflowing lag * (load + weight) by distributing
+			 * the division:
+			 *
+			 *   lag * (load + weight) / load
+			 *     = lag + lag * weight / load
+			 */
+			lag += div64_long(lag * weight, load);
+		}
 
 		/*
 		 * A heavy entity (relative to the tree) will pull the

base-commit: 6812ce4e4379ffc99c52401ec28f0d7ffbc36206