Re: [PATCH net-next] net: fix up truesize of cloned skb in skb_prepare_for_shift()

[Christoph Paasch]

On Mon, Feb 1, 2021 at 8:09 AM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> Avoid the assumption that ksize(kmalloc(S)) == ksize(kmalloc(S)): when
> cloning an skb, save and restore truesize after pskb_expand_head(). This
> can occur if the allocator decides to service an allocation of the same
> size differently (e.g. use a different size class, or pass the
> allocation on to KFENCE).
>
> Because truesize is used for bookkeeping (such as sk_wmem_queued), a
> modified truesize of a cloned skb may result in corrupt bookkeeping and
> relevant warnings (such as in sk_stream_kill_queues()).
>
> Link: https://lkml.kernel.org/r/X9JR/J6dMMOy1obu@xxxxxxxxxxxxxxxx
> Reported-by: syzbot+7b99aafdcc2eedea6178@xxxxxxxxxxxxxxxxxxxxxxxxx
> Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> ---
>  net/core/skbuff.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 2af12f7e170c..3787093239f5 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -3289,7 +3289,19 @@ EXPORT_SYMBOL(skb_split);
>   */
>  static int skb_prepare_for_shift(struct sk_buff *skb)
>  {
> -       return skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
> +       int ret = 0;
> +
> +       if (skb_cloned(skb)) {
> +               /* Save and restore truesize: pskb_expand_head() may reallocate
> +                * memory where ksize(kmalloc(S)) != ksize(kmalloc(S)), but we
> +                * cannot change truesize at this point.
> +                */
> +               unsigned int save_truesize = skb->truesize;
> +
> +               ret = pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
> +               skb->truesize = save_truesize;
> +       }
> +       return ret;

just a few days ago we found out that this also fixes a syzkaller
issue on MPTCP (https://github.com/multipath-tcp/mptcp_net-next/issues/136).
I confirmed that this patch fixes the issue for us as well:

Tested-by: Christoph Paasch <christoph.paasch@xxxxxxxxx>





>  }
>
>  /**
>
> base-commit: 14e8e0f6008865d823a8184a276702a6c3cbef3d
> --
> 2.30.0.365.g02bc693789-goog
>