netdev - Re: [PATCH net] tcp: take care of mixed splice()/sendmsg(MSG

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Date:   Fri, 4 Feb 2022 09:39:24 -0500
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        syzbot <syzkaller@...glegroups.com>,
        Talal Ahmad <talalahmad@...gle.com>,
        Arjun Roy <arjunroy@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] tcp: take care of mixed splice()/sendmsg(MSG_ZEROCOPY)
 case

On Thu, Feb 3, 2022 at 5:55 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> syzbot found that mixing sendpage() and sendmsg(MSG_ZEROCOPY)
> calls over the same TCP socket would again trigger the
> infamous warning in inet_sock_destruct()
>
>         WARN_ON(sk_forward_alloc_get(sk));
>
> While Talal took into account a mix of regular copied data
> and MSG_ZEROCOPY one in the same skb, the sendpage() path
> has been forgotten.
>
> We want the charging to happen for sendpage(), because
> pages could be coming from a pipe. What is missing is the
> downgrading of pure zerocopy status to make sure
> sk_forward_alloc will stay synced.
>
> Add tcp_downgrade_zcopy_pure() helper so that we can
> use it from the two callers.
>
> Fixes: 9b65b17db723 ("net: avoid double accounting for pure zerocopy skbs")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: syzbot <syzkaller@...glegroups.com>
> Cc: Talal Ahmad <talalahmad@...gle.com>
> Cc: Arjun Roy <arjunroy@...gle.com>
> Cc: Willem de Bruijn <willemb@...gle.com>
> Cc: Soheil Hassas Yeganeh <soheil@...gle.com>

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Nice catch!

> ---
>  net/ipv4/tcp.c | 33 +++++++++++++++++++--------------
>  1 file changed, 19 insertions(+), 14 deletions(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index bdf108f544a45a2aa24bc962fb81dfd0ca1e0682..e1f259da988df7493ce7d71ad8743ec5025e4e7c 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -937,6 +937,22 @@ void tcp_remove_empty_skb(struct sock *sk)
>         }
>  }
>
> +/* skb changing from pure zc to mixed, must charge zc */
> +static int tcp_downgrade_zcopy_pure(struct sock *sk, struct sk_buff *skb)
> +{
> +       if (unlikely(skb_zcopy_pure(skb))) {
> +               u32 extra = skb->truesize -
> +                           SKB_TRUESIZE(skb_end_offset(skb));
> +
> +               if (!sk_wmem_schedule(sk, extra))
> +                       return ENOMEM;
> +
> +               sk_mem_charge(sk, extra);
> +               skb_shinfo(skb)->flags &= ~SKBFL_PURE_ZEROCOPY;
> +       }
> +       return 0;
> +}
> +
>  static struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags,
>                                       struct page *page, int offset, size_t *size)
>  {
> @@ -972,7 +988,7 @@ static struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags,
>                 tcp_mark_push(tp, skb);
>                 goto new_segment;
>         }
> -       if (!sk_wmem_schedule(sk, copy))
> +       if (tcp_downgrade_zcopy_pure(sk, skb) || !sk_wmem_schedule(sk, copy))
>                 return NULL;
>
>         if (can_coalesce) {
> @@ -1320,19 +1336,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>
>                         copy = min_t(int, copy, pfrag->size - pfrag->offset);
>
> -                       /* skb changing from pure zc to mixed, must charge zc */
> -                       if (unlikely(skb_zcopy_pure(skb))) {
> -                               u32 extra = skb->truesize -
> -                                           SKB_TRUESIZE(skb_end_offset(skb));
> -
> -                               if (!sk_wmem_schedule(sk, extra))
> -                                       goto wait_for_space;
> -
> -                               sk_mem_charge(sk, extra);
> -                               skb_shinfo(skb)->flags &= ~SKBFL_PURE_ZEROCOPY;
> -                       }
> -
> -                       if (!sk_wmem_schedule(sk, copy))
> +                       if (tcp_downgrade_zcopy_pure(sk, skb) ||
> +                           !sk_wmem_schedule(sk, copy))
>                                 goto wait_for_space;
>
>                         err = skb_copy_to_page_nocache(sk, &msg->msg_iter, skb,
> --
> 2.35.0.263.gb82422642f-goog
>