[PATCH v2 07/63] skbuff: Switch structure bounds to struct_group()

Kees Cook
2021-08-18T06:07:13Z

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@zx2c4.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Jonathan Lemon <jonathan.lemon@gmail.com>
Cc: Alexander Lobakin <alobakin@pm.me>
Cc: Jakub Sitnicki <jakub@cloudflare.com>
Cc: Marco Elver <elver@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: wireguard@lists.zx2c4.com
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.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