[PATCH net-next V2] net: fix kfree_skb_list use of skb_mark_not_on_list

[Jesper Dangaard Brouer <brouer@redhat.com>]

A bug was introduced by commit eedade12f4cb ("net: kfree_skb_list use
kmem_cache_free_bulk"). It unconditionally unlinked the SKB list via
invoking skb_mark_not_on_list().

In this patch we choose to remove the skb_mark_not_on_list() call as it
isn't necessary. It would be possible and correct to call
skb_mark_not_on_list() only when __kfree_skb_reason() returns true,
meaning the SKB is ready to be free'ed, as it calls/check skb_unref().

This fix is needed as kfree_skb_list() is also invoked on skb_shared_info
frag_list (skb_drop_fraglist() calling kfree_skb_list()). A frag_list can
have SKBs with elevated refcnt due to cloning via skb_clone_fraglist(),
which takes a reference on all SKBs in the list. This implies the
invariant that all SKBs in the list must have the same refcnt, when using
kfree_skb_list().

Reported-by: syzbot+c8a2e66e37eee553c4fd@syzkaller.appspotmail.com
Reported-and-tested-by: syzbot+c8a2e66e37eee553c4fd@syzkaller.appspotmail.com
Fixes: eedade12f4cb ("net: kfree_skb_list use kmem_cache_free_bulk")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 net/core/skbuff.c |    2 --
 1 file changed, 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 4e73ab3482b8..180df58e85c7 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -999,8 +999,6 @@ kfree_skb_list_reason(struct sk_buff *segs, enum skb_drop_reason reason)
 	while (segs) {
 		struct sk_buff *next = segs->next;
 
-		skb_mark_not_on_list(segs);
-
 		if (__kfree_skb_reason(segs, reason))
 			kfree_skb_add_bulk(segs, &sa, reason);