skbuff: Switch structure bounds to struct_group()

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 object code changes (outside of WARNs affected by
source line number changes).
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com> # drivers/net/wireguard/*
Link: https://lore.kernel.org/lkml/20210728035006.GD35706@embeddedorSigned-off-by: David S. Miller <davem@davemloft.net>

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;

include/linux/skbuff.h

@@ -811,12 +811,10 @@ 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];
-	/* public: */
+	struct_group(headers,
 
 	/* private: */
 	__u8			__pkt_type_offset[0];
@@ -921,9 +919,7 @@ struct sk_buff {
 	u64			kcov_handle;
 #endif
 
-	/* private: */
-	__u32			headers_end[0];
-	/* public: */
+	); /* end headers group */
 
 	/* These elements must be at the end, see alloc_skb() for details.  */
 	sk_buff_data_t		tail;

net/core/skbuff.c

@@ -992,12 +992,10 @@ 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)
 {
@@ -1009,14 +1007,12 @@ 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);