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/ipv4/igmp.c b/net/ipv4/igmp.c
index 651cdf648ec4..9fdfd9deac11 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -1435,33 +1435,35 @@ static int __ip_mc_check_igmp(struct sk_buff *skb, struct sk_buff **skb_trimmed)
         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;
 }
 
 /**
@@ -1470,7 +1472,7 @@ static int __ip_mc_check_igmp(struct sk_buff *skb, struct sk_buff **skb_trimmed)
  * @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
@@ -1485,7 +1487,8 @@ static int __ip_mc_check_igmp(struct sk_buff *skb, struct sk_buff **skb_trimmed)
  * 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)
 {