Re: [PATCH NET-NEXT] ipv6: skb_expand_head() adjust skb->truesize incorrectly

[Eric Dumazet <eric.dumazet@gmail.com>]

On 8/23/21 10:25 AM, Christoph Paasch wrote:
> Hello,
> 
> On Mon, Aug 23, 2021 at 12:56 AM Vasily Averin <vvs@virtuozzo.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@gmail.com>
>> Signed-off-by: Vasily Averin <vvs@virtuozzo.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;
 }