Subject: Re: [PATCH net] udp: fix a skb extensions leak

From: Florian Westphal <[email-masked]>
Date: Mon, 30 Mar 2020 15:27:59 +0200

Xin Long <[email-masked]> wrote:
> On udp rx path udp_rcv_segment() may do segment where the frag skbs
> will get the header copied from the head skb in skb_segment_list()
> by calling __copy_skb_header(), which could overwrite the frag skbs'
> extensions by __skb_ext_copy() and cause a leak.
> 
> This issue was found after loading esp_offload where a sec path ext
> is set in the skb.
> 
> On udp tx gso path, it works well as the frag skbs' extensions are
> not set. So this issue should be fixed on udp's rx path only and
> release the frag skbs' extensions before going to do segment.
> Reported-by: Xiumei Mu <[email-masked]>
> Fixes: cf329aa42b66 ("udp: cope with UDP GRO packet misdirection")

Hmm, I suspect this bug came in via
3a1296a38d0cf62bffb9a03c585cbd5dbf15d596 , net: Support GRO/GSO fraglist chaining.

I suspect correct fix is:

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 621b4479fee1..7e29590482ce 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3668,6 +3668,7 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,

                skb_push(nskb, -skb_network_offset(nskb) + offset);

+               skb_release_head_state(nskb);
                 __copy_skb_header(nskb, skb);

                skb_headers_offset_update(nskb, skb_headroom(nskb) - skb_headroom(skb));

AFAICS we not only leak reference of extensions, but also skb->dst and skb->_nfct.