[PATCH 02/13] sched_ext: Guard scx_dsq_move() against NULL kit->dsq after failed iter_new

[Tejun Heo <tj@kernel.org>]

bpf_iter_scx_dsq_new() clears kit->dsq on failure and
bpf_iter_scx_dsq_{next,destroy}() guard against that. scx_dsq_move() doesn't -
it dereferences kit->dsq immediately, so a BPF program that calls
scx_bpf_dsq_move[_vtime]() after a failed iter_new oopses the kernel.

Return false if kit->dsq is NULL.

Fixes: 4c30f5ce4f7a ("sched_ext: Implement scx_bpf_dispatch[_vtime]_from_dsq()")
Cc: stable@vger.kernel.org # v6.12+
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 0c435a4612dc..89170a0e5779 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -8055,12 +8055,22 @@ static bool scx_dsq_move(struct bpf_iter_scx_dsq_kern *kit,
 			 struct task_struct *p, u64 dsq_id, u64 enq_flags)
 {
 	struct scx_dispatch_q *src_dsq = kit->dsq, *dst_dsq;
-	struct scx_sched *sch = src_dsq->sched;
+	struct scx_sched *sch;
 	struct rq *this_rq, *src_rq, *locked_rq;
 	bool dispatched = false;
 	bool in_balance;
 	unsigned long flags;
 
+	/*
+	 * The verifier considers an iterator slot initialized on any
+	 * KF_ITER_NEW return, so a BPF program may legally reach here after
+	 * bpf_iter_scx_dsq_new() failed and left @kit->dsq NULL.
+	 */
+	if (unlikely(!src_dsq))
+		return false;
+
+	sch = src_dsq->sched;
+
 	if (!scx_vet_enq_flags(sch, dsq_id, &enq_flags))
 		return false;
 
-- 
2.53.0