[PATCH] net: Fix usage of pskb_trim_rcsum

[Ross Lagerwall]

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.lagerw...@citrix.com>
---
 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(+)

diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
index 62dc564b251d..f22639f0116a 100644
--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c
@@ -445,6 +445,7 @@ 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)
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 93f56fddd92a..95d25b010a25 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3218,6 +3218,7 @@ int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned 
int len);
  *
  *     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)
diff --git a/net/bridge/br_netfilter_ipv6.c b/net/bridge/br_netfilter_ipv6.c
index 94039f588f1d..564710f88f93 100644
--- 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, 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;
diff --git a/net/bridge/netfilter/nft_reject_bridge.c 
b/net/bridge/netfilter/nft_reject_bridge.c
index 08cbed7d940e..419e8edf23ba 100644
--- 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_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;
diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
index 26921f6b3b92..51d8efba6de2 100644
--- a/net/ipv4/ip_input.c
+++ b/net/ipv4/ip_input.c
@@ -488,6 +488,7 @@ static struct sk_buff *ip_rcv_core(struct sk_buff *skb, 
struct net *net)
                goto drop;
        }
 
+       iph = ip_hdr(skb);
        skb->transport_header = skb->network_header + iph->ihl*4;
 
        /* Remove any debris in the socket control block */
-- 
2.17.2