net: introduce skb_poison_list and use in kfree_skb_list

First user of skb_poison_list is in kfree_skb_list_reason, to catch bugs
earlier like introduced in commit eedade12f4 ("net: kfree_skb_list use
kmem_cache_free_bulk"). For completeness mentioned bug have been fixed in
commit f72ff8b81e ("net: fix kfree_skb_list use of skb_mark_not_on_list").

In case of a bug like mentioned commit we would have seen OOPS with:
 general protection fault, probably for non-canonical address 0xdead000000000870
And content of one the registers e.g. R13: dead000000000800

In this case skb->len is at offset 112 bytes (0x70) why fault happens at
 0x800+0x70 = 0x870

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/poison.h

@@ -81,6 +81,9 @@
 /********** net/core/page_pool.c **********/
 #define PP_SIGNATURE		(0x40 + POISON_POINTER_DELTA)
 
+/********** net/core/skbuff.c **********/
+#define SKB_LIST_POISON_NEXT	((void *)(0x800 + POISON_POINTER_DELTA))
+
 /********** kernel/bpf/ **********/
 #define BPF_PTR_POISON ((void *)(0xeB9FUL + POISON_POINTER_DELTA))
 

include/linux/skbuff.h

@@ -1738,6 +1738,13 @@ static inline void skb_mark_not_on_list(struct sk_buff *skb)
 	skb->next = NULL;
 }
 
+static inline void skb_poison_list(struct sk_buff *skb)
+{
+#ifdef CONFIG_DEBUG_NET
+	skb->next = SKB_LIST_POISON_NEXT;
+#endif
+}
+
 /* Iterate through singly-linked GSO fragments of an skb. */
 #define skb_list_walk_safe(first, skb, next_skb)                               \
 	for ((skb) = (first), (next_skb) = (skb) ? (skb)->next : NULL; (skb);  \

net/core/skbuff.c

@@ -1000,8 +1000,10 @@ kfree_skb_list_reason(struct sk_buff *segs, enum skb_drop_reason reason)
 	while (segs) {
 		struct sk_buff *next = segs->next;
 
-		if (__kfree_skb_reason(segs, reason))
+		if (__kfree_skb_reason(segs, reason)) {
+			skb_poison_list(segs);
 			kfree_skb_add_bulk(segs, &sa, reason);
+		}
 
 		segs = next;
 	}