Re: [PATCH] 6lowpan: iphc: reset mac_header after decompress to fix panic

[Alexander Aring <aring@mojatatu.com>]

Hi,

On Tue, Jun 19, 2018 at 04:44:06PM -0700, 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));
>  

I think it's good to make that if the mac_header gets a dangled pointer.
But we don't have a mac header at this point anymore...

There exists also some functionality that the MAC header is not set, I
suppose this can be usefuly for tun like interfaces e.g. RAW IP what we
have here.

skb_mac_header_was_set

which does:

return skb->mac_header != (typeof(skb->mac_header))~0U;

maybe we can set it as (typeof(skb->mac_header))~0U and then everything
will run as far the kernel will not crash anymore.

Question is for me: which upper layer wants access MAC header here on
receive path?
It cannot parsed anyhow because so far I know no upper layer can parse
at the moment 802.15.4 frames (which is a complex format). Maybe over
some header_ops callback?

- Alex