FAILED: patch "[PATCH] sched_ext: Guard scx_dsq_move() against NULL kit->dsq after" failed to apply to 6.18-stable tree

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 4fda9f0e7c950da4fe03cedeb2ac818edf5d03e9
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026051201-cupping-demotion-047d@gregkh' --subject-prefix 'PATCH 6.18.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 4fda9f0e7c950da4fe03cedeb2ac818edf5d03e9 Mon Sep 17 00:00:00 2001
From: Tejun Heo <[email protected]>
Date: Fri, 24 Apr 2026 14:31:35 -1000
Subject: [PATCH] sched_ext: Guard scx_dsq_move() against NULL kit->dsq after
failed iter_new

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.

index 7f991ecb1398..68c67113204f 100644
@@ -8076,12 +8076,22 @@ 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;
 

From: Tejun Heo <[email protected]>

[ Upstream commit 4fda9f0e7c950da4fe03cedeb2ac818edf5d03e9 ]

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.

---
 kernel/sched/ext.c | 8 ++++++++
 1 file changed, 8 insertions(+)

index 177bbf31116d0..5e989409c7715 100644
@@ -5650,6 +5650,14 @@ static bool scx_dsq_move(struct bpf_iter_scx_dsq_kern *kit,
 	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;
+
 	if (!scx_kf_allowed_if_unlocked() &&
 	    !scx_kf_allowed(sch, SCX_KF_DISPATCH))
 		return false;

2.53.0