sched_ext: bpf_iter_scx_dsq_new() should always initialize iterator

BPF programs may call next() and destroy() on BPF iterators even after new()
returns an error value (e.g. bpf_for_each() macro ignores error returns from
new()). bpf_iter_scx_dsq_new() could leave the iterator in an uninitialized
state after an error return causing bpf_iter_scx_dsq_next() to dereference
garbage data. Make bpf_iter_scx_dsq_new() always clear $kit->dsq so that
next() and destroy() become noops.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 650ba21 ("sched_ext: Implement DSQ iterator")
Cc: stable@vger.kernel.org # v6.12+
Acked-by: Andrea Righi <arighi@nvidia.com>

Author: htejun

kernel/sched/ext.c

@@ -6827,6 +6827,12 @@ __bpf_kfunc int bpf_iter_scx_dsq_new(struct bpf_iter_scx_dsq *it, u64 dsq_id,
 	BUILD_BUG_ON(__alignof__(struct bpf_iter_scx_dsq_kern) !=
 		     __alignof__(struct bpf_iter_scx_dsq));
 
+	/*
+	 * next() and destroy() will be called regardless of the return value.
+	 * Always clear $kit->dsq.
+	 */
+	kit->dsq = NULL;
+
 	if (flags & ~__SCX_DSQ_ITER_USER_FLAGS)
 		return -EINVAL;