From: Eric Dumazet <edumazet@google.com>
Date: Mon, 30 Jul 2018 21:50:29 -0700
Subject: ipv4: frags: handle possible skb truesize change
Patch-mainline: v4.18-rc8
Git-commit: 4672694bd4f1aebdab0ad763ae4716e89cb15221
References: CVE-2018-5391 bsc#1103097

ip_frag_queue() might call pskb_pull() on one skb that
is already in the fragment queue.

We need to take care of possible truesize change, or we
might have an imbalance of the netns frags memory usage.

IPv6 is immune to this bug, because RFC5722, Section 4,
amended by Errata ID 3089 states :

  When reassembling an IPv6 datagram, if
  one or more its constituent fragments is determined to be an
  overlapping fragment, the entire datagram (and any constituent
  fragments) MUST be silently discarded.

Fixes: 158f323b9868 ("net: adjust skb->truesize in pskb_expand_head()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Michal Kubecek <mkubecek@suse.cz>

---
 net/ipv4/ip_fragment.c | 5 +++++
 1 file changed, 5 insertions(+)

--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -381,11 +381,16 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb)
 		int i = end - next->ip_defrag_offset; /* overlap is 'i' bytes */
 
 		if (i < next->len) {
+			int delta = -next->truesize;
+
 			/* Eat head of the next overlapped fragment
 			 * and leave the loop. The next ones cannot overlap.
 			 */
 			if (!pskb_pull(next, i))
 				goto err;
+			delta += next->truesize;
+			if (delta)
+				add_frag_mem_limit(qp->q.net, delta);
 			next->ip_defrag_offset += i;
 			qp->q.meat -= i;
 			if (next->ip_summed != CHECKSUM_UNNECESSARY)