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, 19 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index b6a19ca..bf9a5d9 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4022,8 +4022,8 @@ EXPORT_SYMBOL(skb_checksum_setup);
  * 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)
@@ -4032,16 +4032,12 @@ static struct sk_buff *skb_checksum_maybe_trim(struct sk_buff *skb,
 	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;
 
@@ -4066,8 +4062,8 @@ static struct sk_buff *skb_checksum_maybe_trim(struct sk_buff *skb,
  * 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,
@@ -4079,23 +4075,26 @@ struct sk_buff *skb_checksum_trimmed(struct sk_buff *skb,
 
 	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);