netfilter: handle unreadable frags

sashiko reports:
 When an skb with unreadable fragments (such as from devmem TCP, where
 skb_frags_readable(skb) returns false) is processed by the u32 module,
 skb_copy_bits() will safely return a negative error code [..]

xt_u32: bail out with hotdrop in this case.
gather_frags: return -1, just as if we had no fragment header.
nfnetlink_queue: restrict to the linear part.
nfnetlink_log: restrict to the linear part.

v2:
 - skb_zerocopy helpers don't copy readable flag, i.e. nfnetlink_queue
 is broken too
 xt_u32 shouldn't return true if hotdrop was set.

Fixes: 65249feb6b ("net: add support for skbs with unreadable frags")
Cc: stable@vger.kernel.org
Acked-by: Mina Almasry <almasrymina@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

net/ipv6/netfilter/nf_conntrack_reasm.c

@@ -419,7 +419,7 @@ find_prev_fhdr(struct sk_buff *skb, u8 *prevhdrp, int *prevhoff, int *fhoff)
 			return -1;
 		}
 		if (skb_copy_bits(skb, start, &hdr, sizeof(hdr)))
-			BUG();
+			return -1;
 		if (nexthdr == NEXTHDR_AUTH)
 			hdrlen = ipv6_authlen(&hdr);
 		else

net/netfilter/nfnetlink_log.c

@@ -676,7 +676,7 @@ __build_packet_message(struct nfnl_log_net *log,
 			goto nla_put_failure;
 
 		if (skb_copy_bits(skb, 0, nla_data(nla), data_len))
-			BUG();
+			goto nla_put_failure;
 	}
 
 	nlh->nlmsg_len = inst->skb->tail - old_tail;
@@ -698,6 +698,21 @@ static const struct nf_loginfo default_loginfo = {
 	},
 };
 
+static unsigned int nfulnl_get_copy_len(const struct nf_loginfo *li,
+					const struct sk_buff *skb,
+					unsigned int copy_len)
+{
+	unsigned int len = skb->len;
+
+	if ((li->u.ulog.flags & NF_LOG_F_COPY_LEN) &&
+	    li->u.ulog.copy_len < copy_len)
+		copy_len = li->u.ulog.copy_len;
+	if (!skb_frags_readable(skb))
+		len = skb_headlen(skb);
+
+	return min(len, copy_len);
+}
+
 /* log handler for internal netfilter logging api */
 static void
 nfulnl_log_packet(struct net *net,
@@ -790,14 +805,7 @@ nfulnl_log_packet(struct net *net,
 		break;
 
 	case NFULNL_COPY_PACKET:
-		data_len = inst->copy_range;
-		if ((li->u.ulog.flags & NF_LOG_F_COPY_LEN) &&
-		    (li->u.ulog.copy_len < data_len))
-			data_len = li->u.ulog.copy_len;
-
-		if (data_len > skb->len)
-			data_len = skb->len;
-
+		data_len = nfulnl_get_copy_len(li, skb, inst->copy_range);
 		size += nla_total_size(data_len);
 		break;
 

net/netfilter/nfnetlink_queue.c

@@ -690,6 +690,17 @@ static int nfqnl_put_master_ifindex(struct sk_buff *nlskb, int attr,
 }
 #endif
 
+static unsigned int nfqnl_get_data_len(const struct sk_buff *entskb,
+				       unsigned int copy_range)
+{
+	unsigned int data_len = entskb->len;
+
+	if (!skb_frags_readable(entskb))
+		data_len = skb_headlen(entskb);
+
+	return min(data_len, copy_range);
+}
+
 static struct sk_buff *
 nfqnl_build_packet_message(struct net *net, struct nfqnl_instance *queue,
 			   struct nf_queue_entry *entry,
@@ -755,10 +766,7 @@ nfqnl_build_packet_message(struct net *net, struct nfqnl_instance *queue,
 		    nf_queue_checksum_help(entskb))
 			return NULL;
 
-		data_len = READ_ONCE(queue->copy_range);
-		if (data_len > entskb->len)
-			data_len = entskb->len;
-
+		data_len = nfqnl_get_data_len(entskb, READ_ONCE(queue->copy_range));
 		hlen = skb_zerocopy_headlen(entskb);
 		hlen = min_t(unsigned int, hlen, data_len);
 		size += sizeof(struct nlattr) + hlen;

net/netfilter/xt_u32.c

@@ -14,8 +14,8 @@
 #include <linux/netfilter/x_tables.h>
 #include <linux/netfilter/xt_u32.h>
 
-static bool u32_match_it(const struct xt_u32 *data,
-			 const struct sk_buff *skb)
+static int u32_match_it(const struct xt_u32 *data,
+			const struct sk_buff *skb)
 {
 	const struct xt_u32_test *ct;
 	unsigned int testind;
@@ -40,7 +40,8 @@ static bool u32_match_it(const struct xt_u32 *data,
 			return false;
 
 		if (skb_copy_bits(skb, pos, &n, sizeof(n)) < 0)
-			BUG();
+			return -1;
+
 		val   = ntohl(n);
 		nnums = ct->nnums;
 
@@ -68,7 +69,7 @@ static bool u32_match_it(const struct xt_u32 *data,
 
 				if (skb_copy_bits(skb, at + pos, &n,
 						    sizeof(n)) < 0)
-					BUG();
+					return -1;
 				val = ntohl(n);
 				break;
 			}
@@ -90,9 +91,14 @@ static bool u32_match_it(const struct xt_u32 *data,
 static bool u32_mt(const struct sk_buff *skb, struct xt_action_param *par)
 {
 	const struct xt_u32 *data = par->matchinfo;
-	bool ret;
+	int ret;
 
 	ret = u32_match_it(data, skb);
+	if (ret < 0) {
+		par->hotdrop = true;
+		return false;
+	}
+
 	return ret ^ data->invert;
 }