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Date:   Tue, 17 Aug 2021 23:04:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     linux-kernel@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexander Lobakin <alobakin@...me>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Marco Elver <elver@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        wireguard@...ts.zx2c4.com, netdev@...r.kernel.org,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-wireless@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-staging@...ts.linux.dev, linux-block@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-hardening@...r.kernel.org
Subject: [PATCH v2 07/63] 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).

Cc: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: Jakub Kicinski <kuba@...nel.org>
Cc: Jonathan Lemon <jonathan.lemon@...il.com>
Cc: Alexander Lobakin <alobakin@...me>
Cc: Jakub Sitnicki <jakub@...udflare.com>
Cc: Marco Elver <elver@...gle.com>
Cc: Willem de Bruijn <willemb@...gle.com>
Cc: wireguard@...ts.zx2c4.com
Cc: netdev@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>
Link: https://lore.kernel.org/lkml/20210728035006.GD35706@embeddedor
---
 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 6bdb0db3e825..fee9041aa402 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -801,11 +801,10 @@ 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 */
@@ -922,8 +921,8 @@ 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 f9311762cc47..fd5ce57ccce6 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -991,12 +991,10 @@ 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)
 {
@@ -1008,14 +1006,12 @@ 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

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