[PATCH 6.18.y] sched_ext: Guard scx_dsq_move() against NULL kit->dsq after failed iter_new

[Sasha Levin <sashal@kernel.org>]

From: Tejun Heo <tj@kernel.org>

[ 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.

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>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
[ dropped upstream `sch = src_dsq->sched` reordering since stable initializes `sch` from `scx_root` instead ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/sched/ext.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 177bbf31116d0..5e989409c7715 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -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