netdev - Re: [PATCH net-next 2/2] net: convert (struct ubuf_info)->refcnt to refcount

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Message-ID: <CAF=yD-KoztUEn-K5G9mvjE6wpq5+V9opAs5pim5cBr5cdvv3ig@mail.gmail.com>
Date:   Thu, 31 Aug 2017 18:45:47 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next 2/2] net: convert (struct ubuf_info)->refcnt to refcount_t

On Thu, Aug 31, 2017 at 4:30 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> refcount_t type and corresponding API should be
> used instead of atomic_t when the variable is used as
> a reference counter. This allows to avoid accidental
> refcounter overflows that might lead to use-after-free
> situations.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  include/linux/skbuff.h | 5 +++--
>  net/core/skbuff.c      | 8 ++++----
>  2 files changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 7594e19bce622a38dc39c054093c3da15b99b67b..316a92b45351f53709886ee0099cbc83b66f1b15 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -22,6 +22,7 @@
>  #include <linux/cache.h>
>  #include <linux/rbtree.h>
>  #include <linux/socket.h>
> +#include <linux/refcount.h>
>
>  #include <linux/atomic.h>
>  #include <asm/types.h>
> @@ -456,7 +457,7 @@ struct ubuf_info {
>                         u32 bytelen;
>                 };
>         };
> -       atomic_t refcnt;
> +       refcount_t refcnt;
>
>         struct mmpin {
>                 struct user_struct *user;
> @@ -472,7 +473,7 @@ struct ubuf_info *sock_zerocopy_realloc(struct sock *sk, size_t size,
>
>  static inline void sock_zerocopy_get(struct ubuf_info *uarg)
>  {
> -       atomic_inc(&uarg->refcnt);
> +       refcount_inc(&uarg->refcnt);
>  }
>
>  void sock_zerocopy_put(struct ubuf_info *uarg);
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 65b9ca3945f8fd2b1bef4aef5dd774be04e5d128..ed86ca9afd9d8d1ac47983acf6006c179285a612 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -963,7 +963,7 @@ struct ubuf_info *sock_zerocopy_alloc(struct sock *sk, size_t size)
>         uarg->len = 1;
>         uarg->bytelen = size;
>         uarg->zerocopy = 1;
> -       atomic_set(&uarg->refcnt, 1);
> +       refcount_set(&uarg->refcnt, 1);
>         sock_hold(sk);
>
>         return uarg;
> @@ -1086,7 +1086,7 @@ EXPORT_SYMBOL_GPL(sock_zerocopy_callback);
>
>  void sock_zerocopy_put(struct ubuf_info *uarg)
>  {
> -       if (uarg && atomic_dec_and_test(&uarg->refcnt)) {
> +       if (uarg && refcount_dec_and_test(&uarg->refcnt)) {
>                 if (uarg->callback)
>                         uarg->callback(uarg, uarg->zerocopy);
>                 else
> @@ -1108,7 +1108,7 @@ void sock_zerocopy_put_abort(struct ubuf_info *uarg)
>                  * avoid an skb send inside the main loop triggering uarg free.
>                  */
>                 if (sk->sk_type != SOCK_STREAM)
> -                       atomic_inc(&uarg->refcnt);
> +                       refcount_inc(&uarg->refcnt);

This would be a 0 -> 1 transition. It is taken only on the error path from
a socket that does not take an explicit hold at the start of sendmsg.

The code is vestigial, really, as the final patchset only includes tcp.
It is safe to leave as is for now, or I can remove it.

In 1f8b977ab32d ("sock: enable MSG_ZEROCOPY") I also updated
other users of ubuf_info to initialize refcnt. We probably also want to
convert the call in handle_tx in drivers/vhost/net.c.