Re: [PATCH v3 2/3] net: add kcov handle to skb extensions

[Willem de Bruijn]

On Mon, Oct 26, 2020 at 11:11 AM Aleksandr Nogikh
<aleksandrnogikh@xxxxxxxxx> wrote:
>
> From: Aleksandr Nogikh <nogikh@xxxxxxxxxx>
>
> Remote KCOV coverage collection enables coverage-guided fuzzing of the
> code that is not reachable during normal system call execution. It is
> especially helpful for fuzzing networking subsystems, where it is
> common to perform packet handling in separate work queues even for the
> packets that originated directly from the user space.
>
> Enable coverage-guided frame injection by adding kcov remote handle to
> skb extensions. Default initialization in __alloc_skb and
> __build_skb_around ensures that no socket buffer that was generated
> during a system call will be missed.
>
> Code that is of interest and that performs packet processing should be
> annotated with kcov_remote_start()/kcov_remote_stop().
>
> An alternative approach is to determine kcov_handle solely on the
> basis of the device/interface that received the specific socket
> buffer. However, in this case it would be impossible to distinguish
> between packets that originated during normal background network
> processes or were intentionally injected from the user space.
>
> Signed-off-by: Aleksandr Nogikh <nogikh@xxxxxxxxxx>
> --
> v2 -> v3:
> * Reimplemented this change. Now kcov handle is added to skb
> extensions instead of sk_buff.
> v1 -> v2:
> * Updated the commit message.
> ---
>  include/linux/skbuff.h | 31 +++++++++++++++++++++++++++++++
>  net/core/skbuff.c      | 11 +++++++++++
>  2 files changed, 42 insertions(+)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index a828cf99c521..b63d90faa8e9 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -4150,6 +4150,9 @@ enum skb_ext_id {
>  #endif
>  #if IS_ENABLED(CONFIG_MPTCP)
>         SKB_EXT_MPTCP,
> +#endif
> +#if IS_ENABLED(CONFIG_KCOV)
> +       SKB_EXT_KCOV_HANDLE,
>  #endif
>         SKB_EXT_NUM, /* must be last */
>  };
> @@ -4605,5 +4608,33 @@ static inline void skb_reset_redirect(struct sk_buff *skb)
>  #endif
>  }
>
> +#ifdef CONFIG_KCOV
> +
> +static inline void skb_set_kcov_handle(struct sk_buff *skb, const u64 kcov_handle)
> +{
> +       /* No reason to allocate skb extensions to set kcov_handle if kcov_handle is 0. */

If the handle does not need to be set if zero, why then set it if the
skb has extensions?

> +       if (skb_has_extensions(skb) || kcov_handle) {
> +               u64 *kcov_handle_ptr = skb_ext_add(skb, SKB_EXT_KCOV_HANDLE);

skb_ext_add and skb_ext_find are not defined unless CONFIG_SKB_EXTENSIONS.

Perhaps CONFIG_KCOV should be made to select that?




> +
> +               if (kcov_handle_ptr)
> +                       *kcov_handle_ptr = kcov_handle;
> +       }
> +}
> +
> +static inline u64 skb_get_kcov_handle(struct sk_buff *skb)
> +{
> +       u64 *kcov_handle = skb_ext_find(skb, SKB_EXT_KCOV_HANDLE);
> +
> +       return kcov_handle ? *kcov_handle : 0;
> +}
> +
> +#else
> +
> +static inline void skb_set_kcov_handle(struct sk_buff *skb, const u64 kcov_handle) { }
> +
> +static inline u64 skb_get_kcov_handle(struct sk_buff *skb) { return 0; }
> +
> +#endif /* CONFIG_KCOV */