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Message-ID: <61da35a1-e609-02d5-609d-5228e184e43f@virtuozzo.com>
Date: Tue, 24 Aug 2021 20:21:43 +0300
From: Vasily Averin <vvs@...tuozzo.com>
To: Eric Dumazet <eric.dumazet@...il.com>,
Christoph Paasch <christoph.paasch@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>,
Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
David Ahern <dsahern@...nel.org>,
Jakub Kicinski <kuba@...nel.org>,
netdev <netdev@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>, kernel@...nvz.org,
Julian Wiedmann <jwi@...ux.ibm.com>
Subject: Re: [PATCH NET-NEXT] ipv6: skb_expand_head() adjust skb->truesize
incorrectly
On 8/24/21 11:50 AM, Vasily Averin wrote:
> On 8/24/21 1:23 AM, Eric Dumazet wrote:
>> On 8/23/21 2:51 PM, Eric Dumazet wrote:
>>> On 8/23/21 2:45 PM, Eric Dumazet wrote:
>>>> On 8/23/21 10:25 AM, Christoph Paasch wrote:
>>>>> Hello,
>>>>>
>>>>> On Mon, Aug 23, 2021 at 12:56 AM Vasily Averin <vvs@...tuozzo.com> wrote:
>>>>>>
>>>>>> Christoph Paasch reports [1] about incorrect skb->truesize
>>>>>> after skb_expand_head() call in ip6_xmit.
>>>>>> This happen because skb_set_owner_w() for newly clone skb is called
>>>>>> too early, before pskb_expand_head() where truesize is adjusted for
>>>>>> (!skb-sk) case.
>>>>>>
>>>>>> [1] https://lkml.org/lkml/2021/8/20/1082
>>>>>>
>>>>>> Reported-by: Christoph Paasch <christoph.paasch@...il.com>
>>>>>> Signed-off-by: Vasily Averin <vvs@...tuozzo.com>
>>>>>> ---
>>>>>> net/core/skbuff.c | 24 +++++++++++++-----------
>>>>>> 1 file changed, 13 insertions(+), 11 deletions(-)
>>>>>>
>>>>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>>>>> index f931176..508d5c4 100644
>>>>>> --- a/net/core/skbuff.c
>>>>>> +++ b/net/core/skbuff.c
>>>>>> @@ -1803,6 +1803,8 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom)
>>>>>>
>>>>>> struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
>>>>>> {
>>>>>> + struct sk_buff *oskb = skb;
>>>>>> + struct sk_buff *nskb = NULL;
>>>>>> int delta = headroom - skb_headroom(skb);
>>>>>>
>>>>>> if (WARN_ONCE(delta <= 0,
>>>>>> @@ -1811,21 +1813,21 @@ struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
>>>>>>
>>>>>> /* pskb_expand_head() might crash, if skb is shared */
>>>>>> if (skb_shared(skb)) {
>>>>>> - struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
>>>>>> -
>>>>>> - if (likely(nskb)) {
>>>>>> - if (skb->sk)
>>>>>> - skb_set_owner_w(nskb, skb->sk);
>>>>>> - consume_skb(skb);
>>>>>> - } else {
>>>>>> - kfree_skb(skb);
>>>>>> - }
>>>>>> + nskb = skb_clone(skb, GFP_ATOMIC);
>>>>>> skb = nskb;
>>>>>> }
>>>>>> if (skb &&
>>>>>> - pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
>>>>>> - kfree_skb(skb);
>>>>>> + pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC))
>>>>>> skb = NULL;
>>>>>> +
>>>>>> + if (!skb) {
>>>>>> + kfree_skb(oskb);
>>>>>> + if (nskb)
>>>>>> + kfree_skb(nskb);
>>>>>> + } else if (nskb) {
>>>>>> + if (oskb->sk)
>>>>>> + skb_set_owner_w(nskb, oskb->sk);
>>>>>> + consume_skb(oskb);
>>>>>
>>>>> sorry, this does not fix the problem. The syzkaller repro still
>>>>> triggers the WARN.
>>>>>
>>>>> When it happens, the skb in ip6_xmit() is not shared as it comes from
>>>>> __tcp_transmit_skb, where it is skb_clone()'d.
>>>>>
>>>>>
>>>>
>>>> Old code (in skb_realloc_headroom())
>>>> was first calling skb2 = skb_clone(skb, GFP_ATOMIC);
>>>>
>>>> At this point, skb2->sk was NULL
>>>> So pskb_expand_head(skb2, SKB_DATA_ALIGN(delta), 0, ...) was able to tweak skb2->truesize
>>>>
>>>> I would try :
>>>>
>>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>>> index f9311762cc475bd38d87c33e988d7c983b902e56..326749a8938637b044a616cc33b6a19ed191ac41 100644
>>>> --- a/net/core/skbuff.c
>>>> +++ b/net/core/skbuff.c
>>>> @@ -1804,6 +1804,7 @@ EXPORT_SYMBOL(skb_realloc_headroom);
>>>> struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
>>>> {
>>>> int delta = headroom - skb_headroom(skb);
>>>> + struct sk_buff *oskb = NULL;
>>>>
>>>> if (WARN_ONCE(delta <= 0,
>>>> "%s is expecting an increase in the headroom", __func__))
>>>> @@ -1813,19 +1814,21 @@ struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
>>>> if (skb_shared(skb)) {
>>>> struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
>>>>
>>>> - if (likely(nskb)) {
>>>> - if (skb->sk)
>>>> - skb_set_owner_w(nskb, skb->sk);
>>>> - consume_skb(skb);
>>>> - } else {
>>>> + if (unlikely(!nskb)) {
>>>> kfree_skb(skb);
>>>> + return NULL;
>>>> }
>>>> + oskb = skb;
>>>> skb = nskb;
>>>> }
>>>> - if (skb &&
>>>> - pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
>>>> + if (pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
>>>> kfree_skb(skb);
>>>> - skb = NULL;
>>>> + kfree_skb(oskb);
>>>> + return NULL;
>>>> + }
>>>> + if (oskb) {
>>>> + skb_set_owner_w(skb, oskb->sk);
>>>> + consume_skb(oskb);
>>>> }
>>>> return skb;
>>>> }
>>>
>>>
>>> Oh well, probably not going to work.
>>>
>>> We have to find a way to properly increase skb->truesize, even if skb_clone() is _not_ called.
>
> Can we adjust truesize outside pskb_expand_head()?
> Could you please explain why it can be not safe?
Do you mean truesize change should not break balance of sk->sk_wmem_alloc?
>> I also note that current use of skb_set_owner_w(), forcing skb->destructor to sock_wfree()
>> is probably breaking TCP Small queues, since original skb->destructor would be tcp_wfree() or __sock_wfree()
>
> I agree, however as far as I understand it is separate and more global problem.
>
> Thank you,
> Vasily Averin
>
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