Linux: net/llc: avoid BUG_ON() in skb_orphan() (CVE-2017-6345)

[Salvatore Bonaccorso <carnil () debian org>]

Hi

CVE-2017-6345 was assigned by MITRE to the following (via
https://cveform.mitre.org/):

https://git.kernel.org/linus/8b74d439e1697110c5e5c600643e823eb1dd0762

> net/llc: avoid BUG_ON() in skb_orphan()
>
> It seems nobody used LLC since linux-3.12.
>
> Fortunately fuzzers like syzkaller still know how to run this code,
> otherwise it would be no fun.
>
> Setting skb->sk without skb->destructor leads to all kinds of
> bugs, we now prefer to be very strict about it.
>
> Ideally here we would use skb_set_owner() but this helper does not exist yet,
> only CAN seems to have a private helper for that.

The fix was backported to 4.9.13 as well.

Regards,
Salvatore