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

On Wed, Mar 1, 2017 at 9:25 PM, Cong Wang [off-list ref] wrote:

On Wed, Mar 1, 2017 at 3:15 PM, Eric Dumazet [off-list ref] wrote:

quoted

On Wed, Mar 1, 2017 at 3:09 PM, Cong Wang [off-list ref] wrote:

quoted

But I doubt skb_orphan() is the solution here, shouldn't we just
update sk->sk_wmem_alloc with skb->truesize changes?

Is it worth it ? Apart from syszkaller I mean...

We started with something that had a real impact on real workloads.

158f323b9868b59967ad96957c4ca388161be321 net: adjust skb->truesize in
pskb_expand_head()

Note that auditing the stack took me a while.

I don't know how sk refcnt could work correctly without making
sk_wmem_alloc correctly. We certainly could just call skb_orphan()
is we don't need skb->sk any more, probably like the frag case,
but for this case, the neigh one, the skb's sitting in neigh->arp_queue
are not going to be freed unless in failed case, therefore skb->sk
should not be orphaned so early.

There is absolutely no issue in arp/nd case.
Many skbs can sit there and it is fine.
Same with skbs sitting a long time in a qdisc.

Of course we try to not call skb_orphan() unless really needed.

tcp_gso_segment() tries very hard to propagate skb ownership to the segments,
but even something apparently easy like that took some patches before
being done right.

(for details : 0d08c42cf9a71530fef5ebcfe368f38f2dd0476f "tcp: gso: fix
truesize tracking")

conntrack reasm is mostly used in forwarding workloads, where skb->sk
is already NULL.

Are you thinking of a real workload where skb->sk _needs_ to be kept
in ipv6 reasm ?