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@embeddedor
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/wireguard/queueing.h b/drivers/net/wireguard/queueing.h
index 4ef2944..52da5e9 100644 (file)
--- 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 c7889af..eba256a 100644 (file)
--- a/include/linux/skbuff.h
+++ b/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;

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 73720bc..a33247f 100644 (file)
--- a/net/core/skbuff.c
+++ b/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);