Re: [PATCH net-next v4] skb_expand_head() adjust skb->truesize incorrectly

From: Vasily Averin
Date: Thu Sep 02 2021 - 04:32:15 EST

On 9/2/21 10:33 AM, Vasily Averin wrote:
> On 9/2/21 10:13 AM, Vasily Averin wrote:
>> On 9/2/21 7:48 AM, Eric Dumazet wrote:
>>> On 9/1/21 9:32 PM, Eric Dumazet wrote:
>>>> I think you missed netem case, in particular
>>>> skb_orphan_partial() which I already pointed out.
>>>> You can setup a stack of virtual devices (tunnels),
>>>> with a qdisc on them, before ip6_xmit() is finally called...
>>>> Socket might have been closed already.
>>>> To test your patch, you could force a skb_orphan_partial() at the beginning
>>>> of skb_expand_head() (extending code coverage)
>>> To clarify :
>>> It is ok to 'downgrade' an skb->destructor having a ref on sk->sk_wmem_alloc to
>>> something owning a ref on sk->refcnt.
>>> But the opposite operation (ref on sk->sk_refcnt --> ref on sk->sk_wmem_alloc) is not safe.
>> Could you please explain in more details, since I stil have a completely opposite point of view?
>> Every sk referenced in skb have sk_wmem_alloc > 9
>> It is assigned to 1 in sk_alloc and decremented right before last __sk_free(),
>> inside both sk_free() sock_wfree() and __sock_wfree()
>> So it is safe to adjust skb->sk->sk_wmem_alloc,
>> because alive skb keeps reference to alive sk and last one keeps sk_wmem_alloc > 0
>> So any destructor used sk->sk_refcnt will already have sk_wmem_alloc > 0,
>> because last sock_put() calls sk_free().
>> However now I'm not sure in reversed direction.
>> skb_set_owner_w() check !sk_fullsock(sk) and call sock_hold(sk);
>> If sk->sk_refcnt can be 0 here (i.e. after execution of old destructor inside skb_orphan)
>> -- it can be trigger pointed problem:
>> "refcount_add() will trigger a warning (panic under KASAN)".
>> Could you please explain where I'm wrong?
> To clarify:
> I'm agree it is unsafe to call on alive skb:

I badly explained the problem in previous letter, let me repeat once again:

I'm told about this piece of code:
+ } else if (sk && skb->destructor != sock_edemux) {
+ delta = osize - skb_end_offset(skb);
+ if (!is_skb_wmem(skb))
+ skb_set_owner_w(skb, sk);
+ skb->truesize += delta;
+ if (sk_fullsock(sk))
+ refcount_add(delta, &sk->sk_wmem_alloc);

it is called on alive expanded skb and it is incorrect because 2 reasons:

a) if old destructor use ref on sk->sk_wmem_alloc
It can decrease to 0 and release sk.
b) if old descriptor use ref on sk->refcnt and !sk_fullsock(sk)
old decriptor can release last reference and release sk.

We can workaround release of sk by move of
refcount_add(delta, &sk->sk_wmem_alloc) before skb_set_owner_w()

} else if (sk && skb->destructor != sock_edemux) {
delta = osize - skb_end_offset(skb);
refcount_add(delta, &sk->sk_wmem_alloc);
if (!is_skb_wmem(skb))
skb_set_owner_w(skb, sk);
skb->truesize += delta;
if (!sk_fullsock(sk))
refcount_dec(delta, &sk->sk_wmem_alloc);

However it it does not resolve b) completely

oid skb_set_owner_w(struct sk_buff *skb, struct sock *sk)
skb_orphan(skb); <<< old destructor releases last sk->refcnt ...
skb->sk = sk;
if (unlikely(!sk_fullsock(sk))) {
skb->destructor = sock_edemux;
sock_hold(sk); <<<< ...and it trigger wrining/panic

Thank you,
Vasily Averin