6lowpan: iphc: reset mac_header after decompress to fix panic

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 <[email protected]>

---
 net/6lowpan/iphc.c | 1 +
 1 file changed, 1 insertion(+)

net/6lowpan/iphc.c +0 -0

index 6b1042e21656..52fad5dad9f7 100644

Hi,

Quoted 32 lines · Michael Scott

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 <[email protected]>

---
  net/6lowpan/iphc.c | 1 +
  1 file changed, 1 insertion(+)

net/6lowpan/iphc.c +0 -0

index 6b1042e21656..52fad5dad9f7 100644

net/6lowpan/iphc.c int lowpan_header_decompress(struct sk_buff *skb, const struct net_device *dev, · line 776

Quoted 7 lines

net/6lowpan/iphc.c +1 -0

@@ -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

Hello Alexander,

Quoted 5 lines

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

Quoted 1 line

Hi,

Quoted 2 lines · Michael Scott

Hello Alexander,

...

Quoted 6 lines

Quoted 5 lines

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?

Quoted 17 lines

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

Quoted 1 line

"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:

net/6lowpan/iphc.c +1 -0

index 6b1042e21656..e6ec2df3afe0 100644
@@ -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.

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 <[email protected]>

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 <[email protected]>

---
 net/6lowpan/iphc.c | 1 +
 1 file changed, 1 insertion(+)

net/6lowpan/iphc.c +0 -0

index 6b1042e21656..52fad5dad9f7 100644

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.

Signed-off-by Michael Scott <[email protected]>

---
 net/6lowpan/iphc.c | 1 +
 1 file changed, 1 insertion(+)

net/6lowpan/iphc.c +0 -0

index 6b1042e21656..52fad5dad9f7 100644

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 <[email protected]>

---
 net/6lowpan/iphc.c | 1 +
 1 file changed, 1 insertion(+)

net/6lowpan/iphc.c +0 -0

index 6b1042e21656..52fad5dad9f7 100644

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 <[email protected]>

---
net/6lowpan/iphc.c | 1 +

1 file changed, 1 insertion(+)