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Date:   Mon, 11 Oct 2021 10:04:55 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Corentin Noël <corentin.noel@...labora.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] virtio-net: fix for skb_over_panic inside big mode

On Sat, Oct 9, 2021 at 5:18 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
>
> commit 126285651b7f ("Merge ra.kernel.org:/pub/scm/linux/kernel/git/netdev/net")
> accidentally reverted the effect of
> commit 1a8024239da ("virtio-net: fix for skb_over_panic inside big mode")
> on drivers/net/virtio_net.c
>
> As a result, users of crosvm (which is using large packet mode)
> are experiencing crashes with 5.14-rc1 and above that do not
> occur with 5.13.
>
> Crash trace:
>
> [   61.346677] skbuff: skb_over_panic: text:ffffffff881ae2c7 len:3762 put:3762 head:ffff8a5ec8c22000 data:ffff8a5ec8c22010 tail:0xec2 end:0xec0 dev:<NULL>
> [   61.369192] kernel BUG at net/core/skbuff.c:111!
> [   61.372840] invalid opcode: 0000 [#1] SMP PTI
> [   61.374892] CPU: 5 PID: 0 Comm: swapper/5 Not tainted 5.14.0-rc1 linux-v5.14-rc1-for-mesa-ci.tar.bz2 #1
> [   61.376450] Hardware name: ChromiumOS crosvm, BIOS 0
>
> ..
>
> [   61.393635] Call Trace:
> [   61.394127]  <IRQ>
> [   61.394488]  skb_put.cold+0x10/0x10
> [   61.395095]  page_to_skb+0xf7/0x410
> [   61.395689]  receive_buf+0x81/0x1660
> [   61.396228]  ? netif_receive_skb_list_internal+0x1ad/0x2b0
> [   61.397180]  ? napi_gro_flush+0x97/0xe0
> [   61.397896]  ? detach_buf_split+0x67/0x120
> [   61.398573]  virtnet_poll+0x2cf/0x420
> [   61.399197]  __napi_poll+0x25/0x150
> [   61.399764]  net_rx_action+0x22f/0x280
> [   61.400394]  __do_softirq+0xba/0x257
> [   61.401012]  irq_exit_rcu+0x8e/0xb0
> [   61.401618]  common_interrupt+0x7b/0xa0
> [   61.402270]  </IRQ>
>
> See
> https://lore.kernel.org/r/5edaa2b7c2fe4abd0347b8454b2ac032b6694e2c.camel%40collabora.com
> for the report.
>
> Apply the original 1a8024239da ("virtio-net: fix for skb_over_panic inside big mode")
> again, the original logic still holds:
>
> In virtio-net's large packet mode, there is a hole in the space behind
> buf.
>
>     hdr_padded_len - hdr_len
>
> We must take this into account when calculating tailroom.
>
> Cc: Greg KH <gregkh@...uxfoundation.org>
> Fixes: fb32856b16ad ("virtio-net: page_to_skb() use build_skb when there's sufficient tailroom")
> Fixes: 126285651b7f ("Merge ra.kernel.org:/pub/scm/linux/kernel/git/netdev/net")
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> Reported-by: Corentin Noël <corentin.noel@...labora.com>
> Tested-by: Corentin Noël <corentin.noel@...labora.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---

Acked-by: Jason Wang <jasowang@...hat.com>

>
> David, I think we need this in stable, too.
>
>  drivers/net/virtio_net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 096c2ac6b7a6..6b0812f44bbf 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -406,7 +406,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>          * add_recvbuf_mergeable() + get_mergeable_buf_len()
>          */
>         truesize = headroom ? PAGE_SIZE : truesize;
> -       tailroom = truesize - len - headroom;
> +       tailroom = truesize - len - headroom - (hdr_padded_len - hdr_len);
>         buf = p - headroom;
>
>         len -= hdr_len;
> --
> MST
>

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