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

[Alexander Aring]

Hi,

On Mon, Jul 02, 2018 at 12:45:41PM -0700, Michael Scott wrote:
> Hello Alexander,
> 
...
> > 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?
> 
> I was testing a C program which performs NAT64 handling on packets
> destined to a certain IPv6 subnet (64:ff9b::). To do this, the application
> opens a RAW socket like this: sniff_sock = socket(PF_PACKET, SOCK_RAW,
> htons(ETH_P_ALL)); It then sets promiscuous mode and enters a looping call
> of:
> length = recv(sniff_sock, buffer, PACKET_BUFFER, MSG_TRUNC); My host PC
> kernel would then promptly crash on me. (I'm going to purposely avoid the
> obvious point of: this probably isn't the best way to parse packets for
> NAT64 translation as you will get every single packet incoming or outgoing
> on the host.) Turns out, testing the program on an 802.15.4 6lowpan
> interface exposed some of the issues which this mailing list (but not
> myself) is well aware of (no L2 data in the RAW packets) and also led me to
> debugging this patch to stop the kernel crash. TL;DR: To summarize, any
> PF_PACKET SOCK_RAW socket which recv()'s IPv6 data from a 6lowpan node will
> cause this kernel crash eventually (checked on kernel 4.15, 4.16, 4.17 and
> 4.18-rc1). - Mike
> > 

"any PF_PACKET SOCK_RAW" can't be otherwise I would also see it with my
sniffer programs e.g. wireshark or tcpdump which use libpcap.

There need to be some different in the handling. This is what I have
currently in my mind.

I currently not sure how to set skb->mac_header if interface is RAW_IP.
It seems there is an indicator that mac header is not set. Example:

diff --git a/net/6lowpan/iphc.c b/net/6lowpan/iphc.c
index 6b1042e21656..e6ec2df3afe0 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->mac_header = (typeof(skb->mac_header))~0U;
        skb_reset_network_header(skb);
        skb_copy_to_linear_data(skb, &hdr, sizeof(hdr));
 

Maybe we should lookup what skb->mac_header points to on tun interfaces
then do the same.

- Alex