Re: [PATCH 05/64] skbuff: Switch structure bounds to struct_group()

Gustavo A. R. Silva
2021-07-28T03:47:40Z

On Tue, Jul 27, 2021 at 01:57:56PM -0700, Kees Cook wrote:
> In preparation for FORTIFY_SOURCE performing compile-time and run-time
> field bounds checking for memcpy(), memmove(), and memset(), avoid
> intentionally writing across neighboring fields.
>
> Replace the existing empty member position markers "headers_start" and
> "headers_end" with a struct_group(). This will allow memcpy() and sizeof()
> to more easily reason about sizes, and improve readability.
>
> "pahole" shows no size nor member offset changes to struct sk_buff.
> "objdump -d" shows no no meaningful object code changes (i.e. only source
> line number induced differences and optimizations.)
>
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks
--
Gustavo

> ---
> drivers/net/wireguard/queueing.h | 4 +---
> include/linux/skbuff.h | 9 ++++-----
> net/core/skbuff.c | 14 +++++---------
> 3 files changed, 10 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/net/wireguard/queueing.h b/drivers/net/wireguard/queueing.h
> index 4ef2944a68bc..52da5e963003 100644
> --- a/drivers/net/wireguard/queueing.h
> +++ b/drivers/net/wireguard/queueing.h
> @@ -79,9 +79,7 @@ static inline void wg_reset_packet(struct sk_buff *skb, bool encapsulating)
> u8 sw_hash = skb->sw_hash;
> u32 hash = skb->hash;
> skb_scrub_packet(skb, true);
> - memset(&skb->headers_start, 0,
> - offsetof(struct sk_buff, headers_end) -
> - offsetof(struct sk_buff, headers_start));
> + memset(&skb->headers, 0, sizeof(skb->headers));
> if (encapsulating) {
> skb->l4_hash = l4_hash;
> skb->sw_hash = sw_hash;
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index f19190820e63..b4032e9b130e 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -800,11 +800,10 @@ struct sk_buff {
> __u8 active_extensions;
> #endif
>
> - /* fields enclosed in headers_start/headers_end are copied
> + /* Fields enclosed in headers group are copied
> * using a single memcpy() in __copy_skb_header()
> */
> - /* private: */
> - __u32 headers_start[0];
> + struct_group(headers,
> /* public: */
>
> /* if you move pkt_type around you also must adapt those constants */
> @@ -920,8 +919,8 @@ struct sk_buff {
> u64 kcov_handle;
> #endif
>
> - /* private: */
> - __u32 headers_end[0];
> + ); /* end headers group */
> +
> /* public: */
>
> /* These elements must be at the end, see alloc_skb() for details. */
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index fc7942c0dddc..5f29c65507e0 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -987,12 +987,10 @@ void napi_consume_skb(struct sk_buff *skb, int budget)
> }
> EXPORT_SYMBOL(napi_consume_skb);
>
> -/* Make sure a field is enclosed inside headers_start/headers_end section */
> +/* Make sure a field is contained by headers group */
> #define CHECK_SKB_FIELD(field) \
> - BUILD_BUG_ON(offsetof(struct sk_buff, field) < \
> - offsetof(struct sk_buff, headers_start)); \
> - BUILD_BUG_ON(offsetof(struct sk_buff, field) > \
> - offsetof(struct sk_buff, headers_end)); \
> + BUILD_BUG_ON(offsetof(struct sk_buff, field) != \
> + offsetof(struct sk_buff, headers.field)); \
>
> static void __copy_skb_header(struct sk_buff *new, const struct sk_buff *old)
> {
> @@ -1004,14 +1002,12 @@ static void __copy_skb_header(struct sk_buff *new, const struct sk_buff *old)
> __skb_ext_copy(new, old);
> __nf_copy(new, old, false);
>
> - /* Note : this field could be in headers_start/headers_end section
> + /* Note : this field could be in the headers group.
> * It is not yet because we do not want to have a 16 bit hole
> */
> new->queue_mapping = old->queue_mapping;
>
> - memcpy(&new->headers_start, &old->headers_start,
> - offsetof(struct sk_buff, headers_end) -
> - offsetof(struct sk_buff, headers_start));
> + memcpy(&new->headers, &old->headers, sizeof(new->headers));
> CHECK_SKB_FIELD(protocol);
> CHECK_SKB_FIELD(csum);
> CHECK_SKB_FIELD(hash);
> --
> 2.30.2
>