[PATCH net] filter: Account for tail adjustment during pull operations

Extending the tail can have some unexpected side effects if a program is
reading the content beyond the head skb headlen and all the skbs in the
gso frag_list are linear with no head_frag -

  kernel BUG at net/core/skbuff.c:4219!
  pc : skb_segment+0xcf4/0xd2c
  lr : skb_segment+0x63c/0xd2c
  Call trace:
   skb_segment+0xcf4/0xd2c
   __udp_gso_segment+0xa4/0x544
   udp4_ufo_fragment+0x184/0x1c0
   inet_gso_segment+0x16c/0x3a4
   skb_mac_gso_segment+0xd4/0x1b0
   __skb_gso_segment+0xcc/0x12c
   udp_rcv_segment+0x54/0x16c
   udp_queue_rcv_skb+0x78/0x144
   udp_unicast_rcv_skb+0x8c/0xa4
   __udp4_lib_rcv+0x490/0x68c
   udp_rcv+0x20/0x30
   ip_protocol_deliver_rcu+0x1b0/0x33c
   ip_local_deliver+0xd8/0x1f0
   ip_rcv+0x98/0x1a4
   deliver_ptype_list_skb+0x98/0x1ec
   __netif_receive_skb_core+0x978/0xc60

Fix this by marking these skbs as GSO_DODGY so segmentation can handle
the tail updates accordingly.

Fixes: 5293efe62df8 ("bpf: add bpf_skb_change_tail helper")
Signed-off-by: Sean Tranchetti <redacted>
Signed-off-by: Subash Abhinov Kasiviswanathan <redacted>
---
 net/core/filter.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index bb0136e..d5f7f79 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -1654,6 +1654,20 @@ static DEFINE_PER_CPU(struct bpf_scratchpad, bpf_sp);
 static inline int __bpf_try_make_writable(struct sk_buff *skb,
 					  unsigned int write_len)
 {
+	struct sk_buff *list_skb = skb_shinfo(skb)->frag_list;
+
+	if (skb_is_gso(skb) && list_skb && !list_skb->head_frag &&
+	    skb_headlen(list_skb)) {
+		int headlen = skb_headlen(skb);
+		int err = skb_ensure_writable(skb, write_len);
+
+		/* pskb_pull_tail() has occurred */
+		if (!err && headlen != skb_headlen(skb))
+			skb_shinfo(skb)->gso_type |= SKB_GSO_DODGY;
+
+		return err;
+	}
+
 	return skb_ensure_writable(skb, write_len);
 }
 

-- 
2.7.4