Re: net: use-after-free in neigh_timer_handler/sock_wfree

On Wed, Mar 1, 2017 at 1:43 PM, Cong Wang [off-list ref] wrote:

quoted

This one looks very similar to a previous one:
https://groups.google.com/forum/#!topic/syzkaller/BhyN5OFd7sQ

Both happen on raw v6 sockets.

For me, it seems the sk refcnt is not correct, skb should still hold
a refcnt so it should not be freed before kfree_skb() in a timer
handler...

More precisely, after this commit:

commit 2b85a34e911bf483c27cfdd124aeb1605145dc80
Author: Eric Dumazet [off-list ref]
Date:   Thu Jun 11 02:55:43 2009 -0700

    net: No more expensive sock_hold()/sock_put() on each tx

we don't take (old) refcnt any more on TX path, sk_wmem_alloc
is the new refcnt. ;)

So the bug is that skb->truesize is mangled by reassembly unit,
while sbk->sk is tracking sk_wmem_alloc changes in order
to decide when it is safe to free sk.

This is why we need to call skb_orphan(), as we did for IPv4 in
8282f27449bf15548