net: Fix usage of pskb_trim_rcsum

BugLink: https://bugs.launchpad.net/bugs/1818803

[ Upstream commit 6c57f045 ]

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>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/net/ppp/pppoe.c

@@ -442,6 +442,7 @@ static int pppoe_rcv(struct sk_buff *skb, struct net_device *dev,
 	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)

include/linux/skbuff.h

@@ -2803,6 +2803,7 @@ static inline unsigned char *skb_push_rcsum(struct sk_buff *skb,
  *
  *	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)

net/bridge/br_netfilter_ipv6.c

@@ -131,6 +131,7 @@ int br_validate_ipv6(struct net *net, struct sk_buff *skb)
 					 IPSTATS_MIB_INDISCARDS);
 			goto drop;
 		}
+		hdr = ipv6_hdr(skb);
 	}
 	if (hdr->nexthdr == NEXTHDR_HOP && br_nf_check_hbh_len(skb))
 		goto drop;

net/bridge/netfilter/nft_reject_bridge.c

@@ -192,6 +192,7 @@ static bool reject6_br_csum_ok(struct sk_buff *skb, int hook)
 	    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;

net/ipv4/ip_input.c

@@ -444,6 +444,7 @@ int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt,
 		goto drop;
 	}
 
+	iph = ip_hdr(skb);
 	skb->transport_header = skb->network_header + iph->ihl*4;
 
 	/* Remove any debris in the socket control block */