Merge branch 'skb-headroom-slab-out-of-bounds'

Stefano Brivio says:

====================
Fix slab out-of-bounds on insufficient headroom for IPv6 packets

Patch 1/2 fixes a slab out-of-bounds occurring with short SCTP packets over
IPv4 over L2TP over IPv6 on a configuration with relatively low HEADER_MAX.

Patch 2/2 makes sure we avoid writing before the allocated buffer in
neigh_hh_output() in case the headroom is enough for the unaligned hardware
header size, but not enough for the aligned one, and that we warn if we hit
this condition.
====================
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/neighbour.h

@@ -454,6 +454,7 @@ static inline int neigh_hh_bridge(struct hh_cache *hh, struct sk_buff *skb)
 
 static inline int neigh_hh_output(const struct hh_cache *hh, struct sk_buff *skb)
 {
+	unsigned int hh_alen = 0;
 	unsigned int seq;
 	unsigned int hh_len;
 
@@ -461,16 +462,33 @@ static inline int neigh_hh_output(const struct hh_cache *hh, struct sk_buff *skb
 		seq = read_seqbegin(&hh->hh_lock);
 		hh_len = hh->hh_len;
 		if (likely(hh_len <= HH_DATA_MOD)) {
-			/* this is inlined by gcc */
-			memcpy(skb->data - HH_DATA_MOD, hh->hh_data, HH_DATA_MOD);
+			hh_alen = HH_DATA_MOD;
+
+			/* skb_push() would proceed silently if we have room for
+			 * the unaligned size but not for the aligned size:
+			 * check headroom explicitly.
+			 */
+			if (likely(skb_headroom(skb) >= HH_DATA_MOD)) {
+				/* this is inlined by gcc */
+				memcpy(skb->data - HH_DATA_MOD, hh->hh_data,
+				       HH_DATA_MOD);
+			}
 		} else {
-			unsigned int hh_alen = HH_DATA_ALIGN(hh_len);
+			hh_alen = HH_DATA_ALIGN(hh_len);
 
-			memcpy(skb->data - hh_alen, hh->hh_data, hh_alen);
+			if (likely(skb_headroom(skb) >= hh_alen)) {
+				memcpy(skb->data - hh_alen, hh->hh_data,
+				       hh_alen);
+			}
 		}
 	} while (read_seqretry(&hh->hh_lock, seq));
 
-	skb_push(skb, hh_len);
+	if (WARN_ON_ONCE(skb_headroom(skb) < hh_alen)) {
+		kfree_skb(skb);
+		return NET_XMIT_DROP;
+	}
+
+	__skb_push(skb, hh_len);
 	return dev_queue_xmit(skb);
 }
 

net/ipv6/ip6_output.c

@@ -195,37 +195,37 @@ int ip6_xmit(const struct sock *sk, struct sk_buff *skb, struct flowi6 *fl6,
 	const struct ipv6_pinfo *np = inet6_sk(sk);
 	struct in6_addr *first_hop = &fl6->daddr;
 	struct dst_entry *dst = skb_dst(skb);
+	unsigned int head_room;
 	struct ipv6hdr *hdr;
 	u8  proto = fl6->flowi6_proto;
 	int seg_len = skb->len;
 	int hlimit = -1;
 	u32 mtu;
 
-	if (opt) {
-		unsigned int head_room;
+	head_room = sizeof(struct ipv6hdr) + LL_RESERVED_SPACE(dst->dev);
+	if (opt)
+		head_room += opt->opt_nflen + opt->opt_flen;
 
-		/* First: exthdrs may take lots of space (~8K for now)
-		   MAX_HEADER is not enough.
-		 */
-		head_room = opt->opt_nflen + opt->opt_flen;
-		seg_len += head_room;
-		head_room += sizeof(struct ipv6hdr) + LL_RESERVED_SPACE(dst->dev);
-
-		if (skb_headroom(skb) < head_room) {
-			struct sk_buff *skb2 = skb_realloc_headroom(skb, head_room);
-			if (!skb2) {
-				IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
-					      IPSTATS_MIB_OUTDISCARDS);
-				kfree_skb(skb);
-				return -ENOBUFS;
-			}
-			if (skb->sk)
-				skb_set_owner_w(skb2, skb->sk);
-			consume_skb(skb);
-			skb = skb2;
+	if (unlikely(skb_headroom(skb) < head_room)) {
+		struct sk_buff *skb2 = skb_realloc_headroom(skb, head_room);
+		if (!skb2) {
+			IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
+				      IPSTATS_MIB_OUTDISCARDS);
+			kfree_skb(skb);
+			return -ENOBUFS;
 		}
+		if (skb->sk)
+			skb_set_owner_w(skb2, skb->sk);
+		consume_skb(skb);
+		skb = skb2;
+	}
+
+	if (opt) {
+		seg_len += opt->opt_nflen + opt->opt_flen;
+
 		if (opt->opt_flen)
 			ipv6_push_frag_opts(skb, opt, &proto);
+
 		if (opt->opt_nflen)
 			ipv6_push_nfrag_opts(skb, opt, &proto, &first_hop,
 					     &fl6->saddr);