This check is wrong, gcc optimizes it away:

                if ((len -= sizeof(pi)) > len)
			return -EINVAL;

This could be responsible for the BUG. If len is 2 or 3 and TUN_NO_PI
isn't set it underflows. alloc_skb() allocates len + 2, which is 0 or
1 byte. skb_reserve tries to reserve 2 bytes and things explode in
skb_put.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>