Seccomp Tools - Provide Powerful Tools For Seccomp Analysis


Provide powerful tools for seccomp analysis.
This project is targeted to (but not limited to) analyze seccomp sandbox in CTF pwn challenges. Some features might be CTF-specific, but still useful for analyzing seccomp in real-case.

Features
  • Dump - Automatically dumps seccomp-bpf from execution file(s).
  • Disasm - Converts bpf to human readable format.
    • Simple decompile.
    • Display syscall names and arguments when possible.
    • Colorful!
  • Asm - Write seccomp rules is so easy!
  • Emu - Emulates seccomp rules.
  • Supports multi-architectures.

Installation
Available on RubyGems.org!
$ gem install seccomp-tools
If you failed when compiling, try:
sudo apt install gcc ruby-dev
and install seccomp-tools again.

Command Line Interface

seccomp-tools
$ seccomp-tools --help
# Usage: seccomp-tools [--version] [--help] <command> [<options>]
#
# List of commands:
#
# asm Seccomp bpf assembler.
# disasm Disassemble seccomp bpf.
# dump Automatically dump seccomp bpf from execution file(s).
# emu Emulate seccomp rules.
#
# See 'seccomp-tools <command> --help' to read about a specific subcommand.

$ seccomp-tools dump --help
# dump - Automatically dump seccomp bpf from execution file(s).
#
# Usage: seccomp-tools dump [exec] [options]
# -c, --sh-exec <command> Executes the given command (via sh).
# Use this option if want to pass arguments or do pipe things to the execution file.
# e.g. use `-c "./bin > /dev/null"` to dump seccomp without being mixed with stdout.
# -f, --format FORMAT Output format. FORMAT can only be one of <disasm|raw|inspect>.
# Default: disasm
# -l, --limit LIMIT Limit the number of calling "prctl(PR_SET_SECCOMP)".
# The target process will be killed whenever its calling times reaches LIMIT.
# Default: 1
# -o, --output FILE Output result into FILE instead of stdout.
# If multiple seccomp syscalls have been invoked (see --limit),
# results will be written to FILE, FILE_1, FILE_2.. etc.
# For example, "--output out.bpf" and the output files are out.bpf, out_1.bpf, ...

dump
Dumps the seccomp bpf from an execution file. This work is done by the ptrace syscall.
NOTICE: beware of the execution file will be executed.
$ file spec/binary/twctf-2016-diary
# spec/binary/twctf-2016-diary: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/l, for GNU/Linux 2.6.24, BuildID[sha1]=3648e29153ac0259a0b7c3e25537a5334f50107f, not stripped

$ seccomp-tools dump spec/binary/twctf-2016-diary
# line CODE JT JF K
# =================================
# 0000: 0x20 0x00 0x00 0x00000000 A = sys_number
# 0001: 0x15 0x00 0x01 0x00000002 if (A != open) goto 0003
# 0002: 0x06 0x00 0x00 0x00000000 return KILL
# 0003: 0x15 0x00 0x01 0x00000101 if (A != openat) goto 0005
# 0004: 0x06 0x00 0x00 0x00000000 return KILL
# 0005: 0x15 0x00 0x01 0x0000003b if (A != execve) goto 0007
# 0006: 0x06 0x00 0x00 0x00000000 return KILL
# 0007: 0x15 0x00 0x01 0x00000038 if (A != clone) goto 0009
# 0008: 0x06 0x00 0x00 0x00000000 return KILL
# 0009: 0x15 0x00 0x01 0x00000039 if (A != fork) goto 001 1
# 0010: 0x06 0x00 0x00 0x00000000 return KILL
# 0011: 0x15 0x00 0x01 0x0000003a if (A != vfork) goto 0013
# 0012: 0x06 0x00 0x00 0x00000000 return KILL
# 0013: 0x15 0x00 0x01 0x00000055 if (A != creat) goto 0015
# 0014: 0x06 0x00 0x00 0x00000000 return KILL
# 0015: 0x15 0x00 0x01 0x00000142 if (A != execveat) goto 0017
# 0016: 0x06 0x00 0x00 0x00000000 return KILL
# 0017: 0x06 0x00 0x00 0x7fff0000 return ALLOW

$ seccomp-tools dump spec/binary/twctf-2016-diary -f inspect
# "\x20\x00\x00\x00\x00\x00\x00\x00\x15\x00\x00\x01\x02\x00\x00\x00\x06\x00\x00\x00\x00\x00\x00\x00\x15\x00\x00\x01\x01\x01\x00\x00\x06\x00\x00\x00\x00\x00\x00\x00\x15\x00\x00\x01\x3B\x00\x00\x00\x06\x00\x00\x00\x00\x00\x00\x00\x15\x00\x00\x01\x38\x00\x00\x00\x06\x00\x00\x00\x00\x00\x00\x00\x15\x00\x00\x01\x39\x00\x00\x00\x06\x00\x00\x00\x00\x00\x00\x00\x15\x00\x00\x01\x3A\x00\x00\x00\x06\x00\x00\x00\x00\x00\x00\x00\x15\x00\x00\x01\x55\x00\x00\x00\x06\x00 \x00\x00\x00\x00\x00\x00\x15\x00\x00\x01\x42\x01\x00\x00\x06\x00\x00\x00\x00\x00\x00\x00\x06\x00\x00\x00\x00\x00\xFF\x7F"

$ seccomp-tools dump spec/binary/twctf-2016-diary -f raw | xxd
# 00000000: 2000 0000 0000 0000 1500 0001 0200 0000 ...............
# 00000010: 0600 0000 0000 0000 1500 0001 0101 0000 ................
# 00000020: 0600 0000 0000 0000 1500 0001 3b00 0000 ............;...
# 00000030: 0600 0000 0000 0000 1500 0001 3800 0000 ............8...
# 00000040: 0600 0000 0000 0000 1500 0001 3900 0000 ............9...
# 00000050: 0600 0000 0000 0000 1500 0001 3a00 0000 ............:...
# 00000060: 0600 0000 0000 0000 1500 0001 5500 0000 ............U...
# 00000070: 0600 0000 0000 0000 1500 0001 4201 0000 ............B...
# 00000080: 0600 0000 0000 0000 0600 0000 0000 ff7f ................

disasm
Disassembles the seccomp from raw bpf.
$ xxd spec/data/twctf-2016-diary.bpf | head -n 3
# 00000000: 2000 0000 0000 0000 1500 0001 0200 0000 ...............
# 00000010: 0600 0000 0000 0000 1500 0001 0101 0000 ................
# 00000020: 0600 0000 0000 0000 1500 0001 3b00 0000 ............;...

$ seccomp-tools disasm spec/data/twctf-2016-diary.bpf
# line CODE JT JF K
# =================================
# 0000: 0x20 0x00 0x00 0x00000000 A = sys_number
# 0001: 0x15 0x00 0x01 0x00000002 if (A != open) goto 0003
# 0002: 0x06 0x00 0x00 0x00000000 return KILL
# 0003: 0x15 0x00 0x01 0x00000101 if (A != openat) goto 0005
# 0004: 0x06 0x00 0x00 0x00000000 return KILL
# 0005: 0x15 0x00 0x01 0x0000003b if (A != execve) goto 0007
# 0006: 0x06 0x00 0x00 0x00000000 return KILL
# 0007: 0x15 0x00 0x01 0x00000038 if (A != clone) goto 0009
# 0008: 0x06 0x00 0x00 0x00000000 return KILL
# 0009: 0x15 0x00 0x01 0x00000039 if (A != fork) goto 0011
# 0010: 0x06 0x00 0x00 0x00000000 return KILL
# 0011: 0x15 0x00 0x01 0x0000003a if (A != vfork) goto 0013
# 0012: 0x06 0x00 0x00 0x00000000 return KILL
# 0013: 0x15 0x00 0x01 0x00000055 if (A != creat) goto 0015
# 0014: 0x06 0x00 0x00 0x00000000 return KILL
# 0015: 0x15 0x00 0x01 0x00000142 if (A != execveat) goto 0017
# 0016: 0x06 0x00 0x00 0x00000000 return KILL
# 0017: 0x06 0x00 0x00 0x7fff0000 return ALLOW

asm
Assembles the seccomp rules into raw bytes. It's very useful when one wants to write custom seccomp rules.
Supports labels for jumping and uses syscall names directly. See examples below.
$ seccomp-tools asm
# asm - Seccomp bpf assembler.
#
# Usage: seccomp-tools asm IN_FILE [options]
# -o, --output FILE Output result into FILE instead of stdout.
# -f, --format FORMAT Output format. FORMAT can only be one of <inspect|raw|c_array|c_source|assembly>.
# Default: inspect
# -a, --arch ARCH Specify architecture.
# Supported architectures are <amd64|i386>.

# Input file for asm
$ cat spec/data/libseccomp.asm
# # check if arch is X86_64
# A = arch
# A == ARCH_X86_64 ? next : dead
# A = sys_number
# A >= 0x40000000 ? dead : next
# A == write ? ok : next
# A == close ? ok : next
# A == dup ? ok : next
# A == exit ? ok : next
# return ERRNO(5)
# ok:
# return ALLOW
# dead:
# return KILL

$ seccomp-tools asm spec/data/libseccomp.asm
# " \x00\x00\x00\x04\x00\x00\x00\x15\x00\x00\b>\x00\x00\xC0 \x00\x00\x00\x00\x00\x00\x005\x00\x06\x00\x00\x00\[email protected]\x15\x00\x04\x00\x01\x00\x00\x00\x15\x00\x03\x00\x03\x00\x00\x00\x15\x00\x02\x00 \x00\x00\x00\x15\x00\x01\x00<\x00\x00\x00\x06\x00\x00\x00\x05\x00\x05\x00\x06\x00\x00\x00\x00\x00\xFF\x7F\x06\x00\x00\x00\x00\x00\x00\x00"

$ seccomp-tools asm spec/data/libseccomp.asm -f c_source
# #include <linux/seccomp.h>
# #include <stdio.h>
# #include <stdlib.h>
# #include <sys/prctl.h>
#
# static void install_seccomp() {
# static unsigned char filter[] = {32,0,0,0,4,0,0,0,21,0,0,8,62,0,0,192,32,0,0,0,0,0,0,0,53,0,6,0,0,0,0,64,21,0,4,0,1,0,0,0,21,0,3,0,3,0,0,0,21,0,2,0,32,0,0,0,21,0,1,0,60,0,0,0,6,0,0,0,5,0,5,0,6,0,0,0,0,0,255,127,6,0,0,0,0,0,0,0};
# struct prog {
# unsigned short len;
# unsig ned char *filter;
# } rule = {
# .len = sizeof(filter) >> 3,
# .filter = filter
# };
# if(prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) { perror("prctl(PR_SET_NO_NEW_PRIVS)"); exit(2); }
# if(prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &rule) < 0) { perror("prctl(PR_SET_SECCOMP)"); exit(2); }
# }

$ seccomp-tools asm spec/data/libseccomp.asm -f assembly
# install_seccomp:
# push rbp
# mov rbp, rsp
# push 38
# pop rdi
# push 0x1
# pop rsi
# xor eax, eax
# mov al, 0x9d
# syscall
# push 22
# pop rdi
# lea rdx, [rip + _filter]
# push rdx /* .filter */
# push _filter_end - _filter >> 3 /* .len */
# mov rdx, rsp
# push 0x2
# pop rsi
# xor eax, eax
# mov al, 0x9d
# syscall
# leave
# ret
# _filter:
# .ascii "\040\000\000\000\004\000\ 000\000\025\000\000\010\076\000\000\300\040\000\000\000\000\000\000\000\065\000\006\000\000\000\000\100\025\000\004\000\001\000\000\000\025\000\003\000\003\000\000\000\025\000\002\000\040\000\000\000\025\000\001\000\074\000\000\000\006\000\000\000\005\000\005\000\006\000\000\000\000\000\377\177\006\000\000\000\000\000\000\000"
# _filter_end:


# let's asm then disasm!
$ seccomp-tools asm spec/data/libseccomp.asm -f raw | seccomp-tools disasm -
# line CODE JT JF K
# =================================
# 0000: 0x20 0x00 0x00 0x00000004 A = arch
# 0001: 0x15 0x00 0x08 0xc000003e if (A != ARCH_X86_64) goto 0010
# 0002: 0x20 0x00 0x00 0x00000000 A = sys_number
# 0003: 0x35 0x06 0x00 0x40000000 if (A >= 0x40000000) goto 0010
# 0004: 0x15 0x04 0x00 0x00000001 if (A == write) goto 0009
# 0005: 0x15 0x03 0x00 0x00000003 if (A == close) goto 0009
# 0006: 0x15 0x02 0x00 0x00000020 if (A == dup) goto 0009
# 00 07: 0x15 0x01 0x00 0x0000003c if (A == exit) goto 0009
# 0008: 0x06 0x00 0x00 0x00050005 return ERRNO(5)
# 0009: 0x06 0x00 0x00 0x7fff0000 return ALLOW
# 0010: 0x06 0x00 0x00 0x00000000 return KILL

Emu
Emulates seccomp given sys_nr, arg0, arg1, etc.
$ seccomp-tools emu --help
# emu - Emulate seccomp rules.
#
# Usage: seccomp-tools emu [options] BPF_FILE [sys_nr [arg0 [arg1 ... arg5]]]
# -a, --arch ARCH Specify architecture.
# Supported architectures are <amd64|i386>.
# -q, --[no-]quiet Run quietly, only show emulation result.

$ seccomp-tools emu spec/data/libseccomp.bpf write 0x3
# line CODE JT JF K
# =================================
# 0000: 0x20 0x00 0x00 0x00000004 A = arch
# 0001: 0x15 0x00 0x08 0xc000003e if (A != ARCH_X86_64) goto 0010
# 0002: 0x20 0x00 0x00 0x00000000 A = sys_number
# 0003: 0x35 0x06 0x00 0x40000000 if (A >= 0x40000000) goto 0010
# 0004: 0x15 0x04 0x00 0x00000001 if (A == write) goto 0009
# 0005: 0x15 0x03 0x00 0x00000003 if (A == close) goto 0009
# 0006: 0x15 0x02 0x00 0x00000020 if (A == dup) goto 0009
# 0 007: 0x15 0x01 0x00 0x0000003c if (A == exit) goto 0009
# 0008: 0x06 0x00 0x00 0x00050005 return ERRNO(5)
# 0009: 0x06 0x00 0x00 0x7fff0000 return ALLOW
# 0010: 0x06 0x00 0x00 0x00000000 return KILL
#
# return ALLOW at line 0009

Screenshots

Dump


Emu



Development
I recommend to use rbenv for your Ruby environment.

Setup
  • Install bundler
    • $ gem install bundler
  • Clone the source
    • $ git clone https://github.com/david942j/seccomp-tools && cd seccomp-tools
  • Install dependencies
    • $ bundle install

Run tests
$ bundle exec rake

I Need You
Any suggestion or feature request is welcome! Feel free to file an issue or send a pull request. And, if you like this work, I'll be happy to be starred

Disqus Comments