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

[Christoph Paasch]

Hello,

On Mon, Aug 23, 2021 at 12:56 AM Vasily Averin <vvs@xxxxxxxxxxxxx> 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@xxxxxxxxx>
> Signed-off-by: Vasily Averin <vvs@xxxxxxxxxxxxx>
> ---
>  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.


Christoph

>         }
>         return skb;
>  }
> --
> 1.8.3.1
>