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Date: Mon, 23 Aug 2021 15:23:29 -0700
From: Eric Dumazet <eric.dumazet@...il.com>
To: Eric Dumazet <eric.dumazet@...il.com>,
Christoph Paasch <christoph.paasch@...il.com>,
Vasily Averin <vvs@...tuozzo.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 8/23/21 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.
>
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()
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