linux-kernel - Re: [PATCH net-next v2] tun: support NAPI for packets received from batched XDP buffs

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Date:   Mon, 28 Feb 2022 15:26:50 +0800
From:   Harold Huang <baymaxhuang@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:XDP (eXpress Data Path)" <bpf@...r.kernel.org>
Subject: Re: [PATCH net-next v2] tun: support NAPI for packets received from
 batched XDP buffs

Thanks for the suggestions.

On Mon, Feb 28, 2022 at 1:17 PM Jason Wang <jasowang@...hat.com> wrote:
>
> On Mon, Feb 28, 2022 at 12:59 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> >
> >
> > On Sun, Feb 27, 2022 at 8:20 PM Jason Wang <jasowang@...hat.com> wrote:
> >>
> >> On Mon, Feb 28, 2022 at 12:06 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
> >>
> >> > How big n can be ?
> >> >
> >> > BTW I could not find where m->msg_controllen was checked in tun_sendmsg().
> >> >
> >> > struct tun_msg_ctl *ctl = m->msg_control;
> >> >
> >> > if (ctl && (ctl->type == TUN_MSG_PTR)) {
> >> >
> >> >      int n = ctl->num;  // can be set to values in [0..65535]
> >> >
> >> >      for (i = 0; i < n; i++) {
> >> >
> >> >          xdp = &((struct xdp_buff *)ctl->ptr)[i];
> >> >
> >> >
> >> > I really do not understand how we prevent malicious user space from
> >> > crashing the kernel.
> >>
> >> It looks to me the only user for this is vhost-net which limits it to
> >> 64, userspace can't use sendmsg() directly on tap.
> >>
> >
> > Ah right, thanks for the clarification.
> >
> > (IMO, either remove the "msg.msg_controllen = sizeof(ctl);" from handle_tx_zerocopy(), or add sanity checks in tun_sendmsg())
> >
> >
>
> Right, Harold, want to do that?

I am greatly willing to do that. But  I am not quite sure about this.

If we remove the "msg.msg_controllen = sizeof(ctl);" from
handle_tx_zerocopy(), it seems msg.msg_controllen is always 0. What
does it stands for?

I see tap_sendmsg in drivers/net/tap.c also uses msg_controller to
send batched xdp buffers. Do we need to add similar sanity checks to
tap_sendmsg  as tun_sendmsg?