My talk at the GPN 2014 about the Berkely Packet Filter and its internal assembler

1. BPF
All your Packets belong to Me
@_xhr_
xhr@giessen.ccc.de

2. xhr GPN 2014 2
BPF ?

3. xhr GPN 2014 3
tcpdump ?

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NIC
¯
Link-Layer Driver
¯
Protocol Stack
¯
Userland
Packet Flow

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Smart Idea

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NIC
¯
Link-Layer Driver
¯
Filter
¯
Buffer
¯
Userland
Packet Flow

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BPF is rather old...
McCanne. Jacobson.The BSD Packet Filter:
A New Architecture for User-level Packet
Capture. in USENIX. 1993.

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Have you met ...

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tcpdump -i eth0 ip6
That's the filter

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0 ldh [12]
1 jeq #0x86dd jt 2 jf 3
2 ret #65535
3 ret #0
Ethernet Protocol Type
0x86dd == IPv6
Accept Packet
Drop Packet

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Linux got a BPF JIT in 2011
Check net/core/filter.c

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Packet Filter only for
Packets?

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seccomp?

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So, how does this work?

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Attach a filter to a socket

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[...]
struct sock_filter code[] = {
{ 0x28, 0, 0, 0x0000000c },
[...]
};
struct sock_fprog bpf = {
.len = ARRAY_SIZE(code),
.filter = code,
};
sock = socket(PF_PACKET, SOCK_RAW,
htons(ETH_P_ALL));
ret = setsockopt(sock, SOL_SOCKET,
SO_ATTACH_FILTER, &bpf, sizeof(bpf));
[...]

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So, how can I use this?

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Need for Space

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A 32 bit wide accumulator
X 32 bit wide X register
M[] 16 x 32 bit "scratch
memory"

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Some Instructions

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ld*
st*
j*
ret
$alu
Load Instructions
Store Instructions
Jumps
Return
ALU instructions

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Hmm … k. IDE anyone?

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tools/net/
bpf_asm
bpf_dbg

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Demo

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What now?

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Packet Filtering

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Can I haz xt_bpf, plz?

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iptables -A <CHAIN>
-m bpf
--bytecode "…"
-j <TARGET>

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And Why?
Because we can!!1
Full packet control
Fine grained filters

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Q & A
xhr
xhr@giessen.ccc.de
@_xhr_