skbuff: Account for tail adjustment during pull operations

Extending the tail can have some unexpected side effects if a program uses
a helper like BPF_FUNC_skb_pull_data to read partial content beyond the
head skb headlen when 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: 3dcbdb1 ("net: gso: Fix skb_segment splat when splitting gso_size mangled skb having linear-headed frag_list")
Signed-off-by: Sean Tranchetti <[email protected]>
Signed-off-by: Subash Abhinov Kasiviswanathan <[email protected]>
Reviewed-by: Alexander Duyck <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Subash Abhinov Kasiviswanathan

net/core/skbuff.c

@@ -2482,6 +2482,9 @@ void *__pskb_pull_tail(struct sk_buff *skb, int delta)
 				insp = list;
 			} else {
 				/* Eaten partially. */
+				if (skb_is_gso(skb) && !list->head_frag &&
+				    skb_headlen(list))
+					skb_shinfo(skb)->gso_type |= SKB_GSO_DODGY;
 
 				if (skb_shared(list)) {
 					/* Sucks! We need to fork list. :-( */