[PATCH] sched_ext: Reject oversized automatic CID shards

[Hui Su]

Automatic topology packing starts a new shard before adding a core when
the current shard would exceed its target size. However, a first core is
always accepted so that cores remain indivisible. An indivisible core with
more than SCX_CID_SHARD_MAX_CPUS CPUs cannot be represented without
exceeding the hard shard limit.

Per-shard state uses fixed-size inline cmask storage sized for
SCX_CID_SHARD_MAX_CPUS. Passing an oversized shard to alloc_pshard() can
therefore make cmask initialization overwrite the following fields.

Reject an indivisible core larger than SCX_CID_SHARD_MAX_CPUS before adding
it to a shard. Existing non-empty shard splitting already keeps the total
within the hard cap, while an indivisible core may still exceed
cid_shard_size when it fits within the hard limit.

Fixes: e2c841b91bf9 ("sched_ext: Add CID sharding")
Signed-off-by: Hui Su <sh_def@xxxxxxx>
---
 kernel/sched/ext/cid.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/ext/cid.c b/kernel/sched/ext/cid.c
index 4b08866d75f6..3c38650d78d5 100644
--- a/kernel/sched/ext/cid.c
+++ b/kernel/sched/ext/cid.c
@@ -287,6 +287,9 @@ s32 scx_cid_init(struct scx_sched *sch)
 				 * empty shard.
 				 */
 				cids_in_core = cpumask_weight(core_scratch);
+				if (cids_in_core > SCX_CID_SHARD_MAX_CPUS)
+					return -E2BIG;
+
 				max_cores = cores_per_shard + (shard_local < nr_large ? 1 : 0);
 				if (cores_in_shard &&
 				    (cores_in_shard >= max_cores ||