net: Fix usage of pskb_trim_rcsum

rosslagerwall · davem330 · commit 6c57f0458022 · 2019-01-18T14:05:14.000-08:00
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 &lt;ross.lagerwall@citrix.com&gt;
Signed-off-by: David S. Miller &lt;davem@davemloft.net&gt;
diff --git 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
@@ -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
@@ -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
@@ -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
@@ -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 */

Original file line number	Diff line number	Diff line change
`@@ -3218,6 +3218,7 @@ int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len);`
`3218`	`3218`	`*`
`3219`	`3219`	`* This is exactly the same as pskb_trim except that it ensures the`
`3220`	`3220`	`* checksum of received packets are still valid after the operation.`
	`3221`	`+ * It can change skb pointers.`
`3221`	`3222`	`*/`
`3222`	`3223`
`3223`	`3224`	`static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)`
Original file line number	Diff line number	Diff line change
`@@ -131,6 +131,7 @@ int br_validate_ipv6(struct net net, struct sk_buff skb)`
`131`	`131`	`IPSTATS_MIB_INDISCARDS);`
`132`	`132`	`goto drop;`
`133`	`133`	`}`
	`134`	`+ hdr = ipv6_hdr(skb);`
`134`	`135`	`}`
`135`	`136`	`if (hdr->nexthdr == NEXTHDR_HOP && br_nf_check_hbh_len(skb))`
`136`	`137`	`goto drop;`
Original file line number	Diff line number	Diff line change
`@@ -488,6 +488,7 @@ static struct sk_buff ip_rcv_core(struct sk_buff skb, struct net *net)`
`488`	`488`	`goto drop;`
`489`	`489`	`}`
`490`	`490`
	`491`	`+ iph = ip_hdr(skb);`
`491`	`492`	`skb->transport_header = skb->network_header + iph->ihl*4;`
`492`	`493`
`493`	`494`	`/* Remove any debris in the socket control block */`