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