netdev - Re: [net v3] drivers/net/wan/lapbether: Use needed_headroom instead of hard_header

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Message-ID: <d02996f90f64d55d5c5e349560bfde46@dev.tdt.de>
Date:   Tue, 04 Aug 2020 14:43:46 +0200
From:   Martin Schiller <ms@....tdt.de>
To:     Xie He <xie.he.0141@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-x25@...r.kernel.org,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Brian Norris <briannorris@...omium.org>,
        netdev-owner@...r.kernel.org
Subject: Re: [net v3] drivers/net/wan/lapbether: Use needed_headroom instead
 of hard_header_len

On 2020-08-02 21:50, Xie He wrote:
> In net/packet/af_packet.c, the function packet_snd first reserves a
> headroom of length (dev->hard_header_len + dev->needed_headroom).
> Then if the socket is a SOCK_DGRAM socket, it calls dev_hard_header,
> which calls dev->header_ops->create, to create the link layer header.
> If the socket is a SOCK_RAW socket, it "un-reserves" a headroom of
> length (dev->hard_header_len), and assumes the user to provide the
> appropriate link layer header.
> 
> So according to the logic of af_packet.c, dev->hard_header_len should
> be the length of the header that would be created by
> dev->header_ops->create.
> 
> However, this driver doesn't provide dev->header_ops, so logically
> dev->hard_header_len should be 0.
> 
> So we should use dev->needed_headroom instead of dev->hard_header_len
> to request necessary headroom to be allocated.

I'm not an expert in the field, but after reading the commit message and
the previous comments, I'd say that makes sense.

> 
> This change fixes kernel panic when this driver is used with AF_PACKET
> SOCK_RAW sockets. Call stack when panic:
> 
> [  168.399197] skbuff: skb_under_panic: text:ffffffff819d95fb len:20
> put:14 head:ffff8882704c0a00 data:ffff8882704c09fd tail:0x11 end:0xc0
> dev:veth0
> ...
> [  168.399255] Call Trace:
> [  168.399259]  skb_push.cold+0x14/0x24
> [  168.399262]  eth_header+0x2b/0xc0
> [  168.399267]  lapbeth_data_transmit+0x9a/0xb0 [lapbether]
> [  168.399275]  lapb_data_transmit+0x22/0x2c [lapb]
> [  168.399277]  lapb_transmit_buffer+0x71/0xb0 [lapb]
> [  168.399279]  lapb_kick+0xe3/0x1c0 [lapb]
> [  168.399281]  lapb_data_request+0x76/0xc0 [lapb]
> [  168.399283]  lapbeth_xmit+0x56/0x90 [lapbether]
> [  168.399286]  dev_hard_start_xmit+0x91/0x1f0
> [  168.399289]  ? irq_init_percpu_irqstack+0xc0/0x100
> [  168.399291]  __dev_queue_xmit+0x721/0x8e0
> [  168.399295]  ? packet_parse_headers.isra.0+0xd2/0x110
> [  168.399297]  dev_queue_xmit+0x10/0x20
> [  168.399298]  packet_sendmsg+0xbf0/0x19b0
> ......

Shouldn't this kernel panic be intercepted by a skb_cow() before the
skb_push() in lapbeth_data_transmit()?

> 
> Additional change:
> When sending, check skb->len to ensure the 1-byte pseudo header is
> present before reading it.
> 
> Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> Cc: Brian Norris <briannorris@...omium.org>
> Signed-off-by: Xie He <xie.he.0141@...il.com>
> ---
> 
> Change from v2:
> Added skb->len check when sending.
> 
> Change from v1:
> None
> 
> ---
>  drivers/net/wan/lapbether.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/wan/lapbether.c b/drivers/net/wan/lapbether.c
> index b2868433718f..8a3f7ba36f7e 100644
> --- a/drivers/net/wan/lapbether.c
> +++ b/drivers/net/wan/lapbether.c
> @@ -157,6 +157,9 @@ static netdev_tx_t lapbeth_xmit(struct sk_buff 
> *skb,
>  	if (!netif_running(dev))
>  		goto drop;
> 
> +	if (skb->len < 1)
> +		goto drop;
> +
>  	switch (skb->data[0]) {
>  	case X25_IFACE_DATA:
>  		break;
> @@ -305,6 +308,7 @@ static void lapbeth_setup(struct net_device *dev)
>  	dev->netdev_ops	     = &lapbeth_netdev_ops;
>  	dev->needs_free_netdev = true;
>  	dev->type            = ARPHRD_X25;
> +	dev->hard_header_len = 0;
>  	dev->mtu             = 1000;
>  	dev->addr_len        = 0;
>  }
> @@ -331,7 +335,8 @@ static int lapbeth_new_device(struct net_device 
> *dev)
>  	 * then this driver prepends a length field of 2 bytes,
>  	 * then the underlying Ethernet device prepends its own header.
>  	 */
> -	ndev->hard_header_len = -1 + 3 + 2 + dev->hard_header_len;
> +	ndev->needed_headroom = -1 + 3 + 2 + dev->hard_header_len
> +					   + dev->needed_headroom;
> 
>  	lapbeth = netdev_priv(ndev);
>  	lapbeth->axdev = ndev;