bpf: Fix NULL deref in __list_del_clearprev for flush_node

#syz test

Hi,

This patch fixes a NULL pointer dereference in the BPF subsystem that occurs
when __list_del_clearprev() is called on an already-cleared flush_node list_head.

The fix includes two parts:
1. Properly initialize the flush_node list_head during per-CPU bulk queue allocation
   using INIT_LIST_HEAD(&bq->flush_node)
2. Add defensive checks before calling __list_del_clearprev() to ensure the node
   is actually in the list by checking if (bq->flush_node.prev)

According to the __list_del_clearprev documentation in include/linux/list.h,
'The code that uses this needs to check the node 'prev' pointer instead of calling list_empty()'.

This patch fixes the following syzbot-reported issue:
https://syzkaller.appspot.com/bug?extid=2b3391f44313b3983e91

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=2b3391f44313b3983e91
Signed-off-by: DLpang <[email protected]>
Reported-by: [email protected]
Tested-by: [email protected]

Author: DLpang

kernel/bpf/cpumap.c

@@ -450,6 +450,7 @@ __cpu_map_entry_alloc(struct bpf_map *map, struct bpf_cpumap_val *value,
 
 	for_each_possible_cpu(i) {
 		bq = per_cpu_ptr(rcpu->bulkq, i);
+		INIT_LIST_HEAD(&bq->flush_node);
 		bq->obj = rcpu;
 	}
 
@@ -742,7 +743,8 @@ static void bq_flush_to_queue(struct xdp_bulk_queue *bq)
 	bq->count = 0;
 	spin_unlock(&q->producer_lock);
 
-	__list_del_clearprev(&bq->flush_node);
+	if (bq->flush_node.prev)
+		__list_del_clearprev(&bq->flush_node);
 
 	/* Feedback loop via tracepoints */
 	trace_xdp_cpumap_enqueue(rcpu->map_id, processed, drops, to_cpu);

kernel/bpf/devmap.c

@@ -428,7 +428,8 @@ void __dev_flush(struct list_head *flush_list)
 		bq_xmit_all(bq, XDP_XMIT_FLUSH);
 		bq->dev_rx = NULL;
 		bq->xdp_prog = NULL;
-		__list_del_clearprev(&bq->flush_node);
+		if (bq->flush_node.prev)
+			__list_del_clearprev(&bq->flush_node);
 	}
 }
 

net/xdp/xsk.c

@@ -406,7 +406,8 @@ void __xsk_map_flush(struct list_head *flush_list)
 
 	list_for_each_entry_safe(xs, tmp, flush_list, flush_node) {
 		xsk_flush(xs);
-		__list_del_clearprev(&xs->flush_node);
+		if (xs->flush_node.prev)
+			__list_del_clearprev(&xs->flush_node);
 	}
 }