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 <bblanco@plumgrid.com>
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 18 insertions, 15 deletions

diff --git a/net/ipv6/mcast_snoop.c b/net/ipv6/mcast_snoop.c
index df8afe5ab31e..9405b04eecc6 100644
--- a/net/ipv6/mcast_snoop.c
+++ b/net/ipv6/mcast_snoop.c
@@ -143,34 +143,36 @@ static int __ipv6_mc_check_mld(struct sk_buff *skb,
 	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;
 }
 
 /**
@@ -179,7 +181,7 @@ static int __ipv6_mc_check_mld(struct sk_buff *skb,
  * @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
@@ -194,7 +196,8 @@ static int __ipv6_mc_check_mld(struct sk_buff *skb,
  * 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)
 {