net: fix wrong skb_get() usage / crash in IGMP/MLD parsing code

The recent refactoring of the IGMP and MLD parsing code into
ipv6_mc_check_mld() / ip_mc_check_igmp() introduced a potential crash /
BUG() invocation for bridges:

I wrongly assumed that skb_get() could be used as a simple reference
counter for an skb which is not the case. skb_get() bears additional
semantics, a user count. This leads to a BUG() invocation in
pskb_expand_head() / kernel panic if pskb_may_pull() is called on an skb
with a user count greater than one - unfortunately the refactoring did
just that.

Fixing this by removing the skb_get() call and changing the API: The
caller of ipv6_mc_check_mld() / ip_mc_check_igmp() now needs to
additionally check whether the returned skb_trimmed is a clone.

Fixes: 9afd85c9e455 ("net: Export IGMP/MLD message validation code")
Reported-by: Brenden Blanco <[e-mail]>
Signed-off-by: Linus Lüssing <[e-mail]>
Acked-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/bridge/br_multicast.c

···
 		break;
 	}
 
-	if (skb_trimmed)
+	if (skb_trimmed && skb_trimmed != skb)
 		kfree_skb(skb_trimmed);
 
 	return err;
···
 		break;
 	}
 
-	if (skb_trimmed)
+	if (skb_trimmed && skb_trimmed != skb)
 		kfree_skb(skb_trimmed);
 
 	return err;

net/core/skbuff.c

···
  * Otherwise returns the provided skb. Returns NULL in error cases
  * (e.g. transport_len exceeds skb length or out-of-memory).
  *
- * Caller needs to set the skb transport header and release the returned skb.
- * Provided skb is consumed.
+ * Caller needs to set the skb transport header and free any returned skb if it
+ * differs from the provided skb.
  */
 static struct sk_buff *skb_checksum_maybe_trim(struct sk_buff *skb,
 					       unsigned int transport_len)
···
 	unsigned int len = skb_transport_offset(skb) + transport_len;
 	int ret;
 
-	if (skb->len < len) {
-		kfree_skb(skb);
+	if (skb->len < len)
 		return NULL;
-	} else if (skb->len == len) {
+	else if (skb->len == len)
 		return skb;
-	}
 
 	skb_chk = skb_clone(skb, GFP_ATOMIC);
-	kfree_skb(skb);
-
 	if (!skb_chk)
 		return NULL;
 
···
  * If the skb has data beyond the given transport length, then a
  * trimmed & cloned skb is checked and returned.
  *
- * Caller needs to set the skb transport header and release the returned skb.
- * Provided skb is consumed.
+ * Caller needs to set the skb transport header and free any returned skb if it
+ * differs from the provided skb.
  */
 struct sk_buff *skb_checksum_trimmed(struct sk_buff *skb,
 				     unsigned int transport_len,
···
 
 	skb_chk = skb_checksum_maybe_trim(skb, transport_len);
 	if (!skb_chk)
-		return NULL;
+		goto err;
 
-	if (!pskb_may_pull(skb_chk, offset)) {
-		kfree_skb(skb_chk);
-		return NULL;
-	}
+	if (!pskb_may_pull(skb_chk, offset))
+		goto err;
 
 	__skb_pull(skb_chk, offset);
 	ret = skb_chkf(skb_chk);
 	__skb_push(skb_chk, offset);
 
-	if (ret) {
-		kfree_skb(skb_chk);
-		return NULL;
-	}
+	if (ret)
+		goto err;
 
 	return skb_chk;
+
+err:
+	if (skb_chk && skb_chk != skb)
+		kfree_skb(skb_chk);
+
+	return NULL;
+
 }
 EXPORT_SYMBOL(skb_checksum_trimmed);
 

net/ipv4/igmp.c

···
 	struct sk_buff *skb_chk;
 	unsigned int transport_len;
 	unsigned int len = skb_transport_offset(skb) + sizeof(struct igmphdr);
-	int ret;
+	int ret = -EINVAL;
 
 	transport_len = ntohs(ip_hdr(skb)->tot_len) - ip_hdrlen(skb);
 
-	skb_get(skb);
 	skb_chk = skb_checksum_trimmed(skb, transport_len,
 				       ip_mc_validate_checksum);
 	if (!skb_chk)
-		return -EINVAL;
+		goto err;
 
-	if (!pskb_may_pull(skb_chk, len)) {
-		kfree_skb(skb_chk);
-		return -EINVAL;
-	}
+	if (!pskb_may_pull(skb_chk, len))
+		goto err;
 
 	ret = ip_mc_check_igmp_msg(skb_chk);
-	if (ret) {
-		kfree_skb(skb_chk);
-		return ret;
-	}
+	if (ret)
+		goto err;
 
 	if (skb_trimmed)
 		*skb_trimmed = skb_chk;
-	else
+	/* free now unneeded clone */
+	else if (skb_chk != skb)
 		kfree_skb(skb_chk);
 
-	return 0;
+	ret = 0;
+
+err:
+	if (ret && skb_chk && skb_chk != skb)
+		kfree_skb(skb_chk);
+
+	return ret;
 }
 
 /**
···
  * @skb_trimmed: to store an skb pointer trimmed to IPv4 packet tail (optional)
  *
  * Checks whether an IPv4 packet is a valid IGMP packet. If so sets
- * skb network and transport headers accordingly and returns zero.
+ * skb transport header accordingly and returns zero.
  *
  * -EINVAL: A broken packet was detected, i.e. it violates some internet
  *  standard
···
  * to leave the original skb and its full frame unchanged (which might be
  * desirable for layer 2 frame jugglers).
  *
- * The caller needs to release a reference count from any returned skb_trimmed.
+ * Caller needs to set the skb network header and free any returned skb if it
+ * differs from the provided skb.
  */
 int ip_mc_check_igmp(struct sk_buff *skb, struct sk_buff **skb_trimmed)
 {

net/ipv6/mcast_snoop.c

···
 	struct sk_buff *skb_chk = NULL;
 	unsigned int transport_len;
 	unsigned int len = skb_transport_offset(skb) + sizeof(struct mld_msg);
-	int ret;
+	int ret = -EINVAL;
 
 	transport_len = ntohs(ipv6_hdr(skb)->payload_len);
 	transport_len -= skb_transport_offset(skb) - sizeof(struct ipv6hdr);
 
-	skb_get(skb);
 	skb_chk = skb_checksum_trimmed(skb, transport_len,
 				       ipv6_mc_validate_checksum);
 	if (!skb_chk)
-		return -EINVAL;
+		goto err;
 
-	if (!pskb_may_pull(skb_chk, len)) {
-		kfree_skb(skb_chk);
-		return -EINVAL;
-	}
+	if (!pskb_may_pull(skb_chk, len))
+		goto err;
 
 	ret = ipv6_mc_check_mld_msg(skb_chk);
-	if (ret) {
-		kfree_skb(skb_chk);
-		return ret;
-	}
+	if (ret)
+		goto err;
 
 	if (skb_trimmed)
 		*skb_trimmed = skb_chk;
-	else
+	/* free now unneeded clone */
+	else if (skb_chk != skb)
 		kfree_skb(skb_chk);
 
-	return 0;
+	ret = 0;
+
+err:
+	if (ret && skb_chk && skb_chk != skb)
+		kfree_skb(skb_chk);
+
+	return ret;
 }
 
 /**
···
  * @skb_trimmed: to store an skb pointer trimmed to IPv6 packet tail (optional)
  *
  * Checks whether an IPv6 packet is a valid MLD packet. If so sets
- * skb network and transport headers accordingly and returns zero.
+ * skb transport header accordingly and returns zero.
  *
  * -EINVAL: A broken packet was detected, i.e. it violates some internet
  *  standard
···
  * to leave the original skb and its full frame unchanged (which might be
  * desirable for layer 2 frame jugglers).
  *
- * The caller needs to release a reference count from any returned skb_trimmed.
+ * Caller needs to set the skb network header and free any returned skb if it
+ * differs from the provided skb.
  */
 int ipv6_mc_check_mld(struct sk_buff *skb, struct sk_buff **skb_trimmed)
 {