batman-adv: Even Batman should not dereference NULL pointers

There's a problem in net/batman-adv/unicast.c::frag_send_skb().
dev_alloc_skb() allocates memory and may fail, thus returning NULL. If
this happens we'll pass a NULL pointer on to skb_split() which in turn
hands it to skb_split_inside_header() from where it gets passed to
skb_put() that lets skb_tail_pointer() play with it and that function
dereferences it. And thus the bat dies.

While I was at it I also moved the call to dev_alloc_skb() above the
assignment to 'unicast_packet' since there's no reason to do that
assignment if the memory allocation fails.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/net/batman-adv/unicast.c b/net/batman-adv/unicast.c
index dc2e28bed844..ee41fef04b21 100644
--- a/net/batman-adv/unicast.c
+++ b/net/batman-adv/unicast.c
@@ -229,10 +229,12 @@ int frag_send_skb(struct sk_buff *skb, struct bat_priv *bat_priv,
         if (!bat_priv->primary_if)
                 goto dropped;
 
-        unicast_packet = (struct unicast_packet *) skb->data;
+        frag_skb = dev_alloc_skb(data_len - (data_len / 2) + ucf_hdr_len);
+        if (!frag_skb)
+                goto dropped;
 
+        unicast_packet = (struct unicast_packet *) skb->data;
         memcpy(&tmp_uc, unicast_packet, uc_hdr_len);
-        frag_skb = dev_alloc_skb(data_len - (data_len / 2) + ucf_hdr_len);
         skb_split(skb, frag_skb, data_len / 2);
 
         if (my_skb_head_push(skb, ucf_hdr_len - uc_hdr_len) < 0 ||