[PATCH 4.20 003/117] net: Fix usage of pskb_trim_rcsum

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.20-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ross Lagerwall <ross.lagerwall@citrix.com>

[ Upstream commit 6c57f0458022298e4da1729c67bd33ce41c14e7a ]

In certain cases, pskb_trim_rcsum() may change skb pointers.
Reinitialize header pointers afterwards to avoid potential
use-after-frees. Add a note in the documentation of
pskb_trim_rcsum(). Found by KASAN.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/net/ppp/pppoe.c                  |    1 +
 include/linux/skbuff.h                   |    1 +
 net/bridge/br_netfilter_ipv6.c           |    1 +
 net/bridge/netfilter/nft_reject_bridge.c |    1 +
 net/ipv4/ip_input.c                      |    1 +
 5 files changed, 5 insertions(+)

--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c
@@ -445,6 +445,7 @@ static int pppoe_rcv(struct sk_buff *skb
 	if (pskb_trim_rcsum(skb, len))
 		goto drop;
 
+	ph = pppoe_hdr(skb);
 	pn = pppoe_pernet(dev_net(dev));
 
 	/* Note that get_item does a sock_hold(), so sk_pppox(po)
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3204,6 +3204,7 @@ int pskb_trim_rcsum_slow(struct sk_buff
  *
  *	This is exactly the same as pskb_trim except that it ensures the
  *	checksum of received packets are still valid after the operation.
+ *	It can change skb pointers.
  */
 
 static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
--- a/net/bridge/br_netfilter_ipv6.c
+++ b/net/bridge/br_netfilter_ipv6.c
@@ -131,6 +131,7 @@ int br_validate_ipv6(struct net *net, st
 					IPSTATS_MIB_INDISCARDS);
 			goto drop;
 		}
+		hdr = ipv6_hdr(skb);
 	}
 	if (hdr->nexthdr == NEXTHDR_HOP && br_nf_check_hbh_len(skb))
 		goto drop;
--- a/net/bridge/netfilter/nft_reject_bridge.c
+++ b/net/bridge/netfilter/nft_reject_bridge.c
@@ -229,6 +229,7 @@ static bool reject6_br_csum_ok(struct sk
 	    pskb_trim_rcsum(skb, ntohs(ip6h->payload_len) + sizeof(*ip6h)))
 		return false;
 
+	ip6h = ipv6_hdr(skb);
 	thoff = ipv6_skip_exthdr(skb, ((u8*)(ip6h+1) - skb->data), &proto, &fo);
 	if (thoff < 0 || thoff >= skb->len || (fo & htons(~0x7)) != 0)
 		return false;
--- a/net/ipv4/ip_input.c
+++ b/net/ipv4/ip_input.c
@@ -489,6 +489,7 @@ static struct sk_buff *ip_rcv_core(struc
 		goto drop;
 	}
 
+	iph = ip_hdr(skb);
 	skb->transport_header = skb->network_header + iph->ihl*4;
 
 	/* Remove any debris in the socket control block */