Hello Alexander,
On 07/02/2018 11:54 AM, Alexander Aring wrote:
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@xxxxxxxxxxxxxxxxxxxxxxx>
---
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?
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
- Alex
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