skbuff: revert "skbuff: remove some unnecessary operation in skb_segment_list()"

the commit 1ddc3229 ("skbuff: remove some unnecessary operation
in skb_segment_list()") introduces an issue very similar to the
one already fixed by commit 53475c5d ("net: fix use-after-free when
UDP GRO with shared fraglist").

If the GSO skb goes though skb_clone() and pskb_expand_head() before
entering skb_segment_list(), the latter  will unshare the frag_list
skbs and will release the old list. With the reverted commit in place,
when skb_segment_list() completes, skb->next points to the just
released list, and later on the kernel will hit UaF.

Note that since commit e0e3070a ("udp: properly complete L4 GRO
over UDP tunnel packet") the critical scenario can be reproduced also
receiving UDP over vxlan traffic with:

NIC (NETIF_F_GRO_FRAGLIST enabled) -> vxlan -> UDP sink

Attaching a packet socket to the NIC will cause skb_clone() and the
tunnel decapsulation will call pskb_expand_head().

Fixes: 1ddc3229 ("skbuff: remove some unnecessary operation in skb_segment_list()")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -3773,13 +3773,13 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
 	unsigned int tnl_hlen = skb_tnl_header_len(skb);
 	unsigned int delta_truesize = 0;
 	unsigned int delta_len = 0;
+	struct sk_buff *tail = NULL;
 	struct sk_buff *nskb, *tmp;
 	int err;
 
 	skb_push(skb, -skb_network_offset(skb) + offset);
 
 	skb_shinfo(skb)->frag_list = NULL;
-	skb->next = list_skb;
 
 	do {
 		nskb = list_skb;
@@ -3797,8 +3797,17 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
 			}
 		}
 
-		if (unlikely(err))
+		if (!tail)
+			skb->next = nskb;
+		else
+			tail->next = nskb;
+
+		if (unlikely(err)) {
+			nskb->next = list_skb;
 			goto err_linearize;
+		}
+
+		tail = nskb;
 
 		delta_len += nskb->len;
 		delta_truesize += nskb->truesize;
@@ -3825,7 +3834,7 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
 
 	skb_gso_reset(skb);
 
-	skb->prev = nskb;
+	skb->prev = tail;
 
 	if (skb_needs_linearize(skb, features) &&
 	    __skb_linearize(skb))