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Subject: Re: [PATCH] 6lowpan: iphc: reset mac_header after decompress to fix
 panic
To:     Michael Scott <michael@opensourcefoundries.com>
Cc:     Alexander Aring <alex.aring@gmail.com>,
        Jukka Rissanen <jukka.rissanen@linux.intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        linux-bluetooth@vger.kernel.org, linux-wpan@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20180619234406.8217-1-michael@opensourcefoundries.com>
From:   Stefan Schmidt <stefan@datenfreihafen.org>
Message-ID: <6ab37806-6ff2-bda5-a457-7dffefa1fe08@datenfreihafen.org>
Date:   Thu, 5 Jul 2018 13:59:27 +0200
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Hello.

On 20.06.2018 01:44, Michael Scott wrote:
> After decompression of 6lowpan socket data, an IPv6 header is inserted
> before the existing socket payload.  After this, we reset the
> network_header value of the skb to account for the difference in payload
> size from prior to decompression + the addition of the IPv6 header.
> 
> However, we fail to reset the mac_header value.
> 
> Leaving the mac_header value untouched here, can cause a calculation
> error in net/packet/af_packet.c packet_rcv() function when an
> AF_PACKET socket is opened in SOCK_RAW mode for use on a 6lowpan
> interface.
> 
> On line 2088, the data pointer is moved backward by the value returned
> from skb_mac_header().  If skb->data is adjusted so that it is before
> the skb->head pointer (which can happen when an old value of mac_header
> is left in place) the kernel generates a panic in net/core/skbuff.c
> line 1717.
> 
> This panic can be generated by BLE 6lowpan interfaces (such as bt0) and
> 802.15.4 interfaces (such as lowpan0) as they both use the same 6lowpan
> sources for compression and decompression.
> 
> Signed-off-by: Michael Scott <michael@opensourcefoundries.com>
> ---
>  net/6lowpan/iphc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/6lowpan/iphc.c b/net/6lowpan/iphc.c
> index 6b1042e21656..52fad5dad9f7 100644
> --- a/net/6lowpan/iphc.c
> +++ b/net/6lowpan/iphc.c
> @@ -770,6 +770,7 @@ int lowpan_header_decompress(struct sk_buff *skb, const struct net_device *dev,
>  		hdr.hop_limit, &hdr.daddr);
>  
>  	skb_push(skb, sizeof(hdr));
> +	skb_reset_mac_header(skb);
>  	skb_reset_network_header(skb);
>  	skb_copy_to_linear_data(skb, &hdr, sizeof(hdr));

This showed no problems in my ieee802154 + 6lowpan tests.

Acked-by: Stefan Schmidt <stefan@datenfreihafen.org>

regards
Stefan Schmidt