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]  []
#
# 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  --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           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 .
#                                      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
# "x20x00x00x00x00x00x00x00x15x00x00x01x02x00x00x00x06x00x00x00x00x00x00x00x15x00x00x01x01x01x00x00x06x00x00x00x00x00x00x00x15x00x00x01x3Bx00x00x00x06x00x00x00x00x00x00x00x15x00x00x01x38x00x00x00x06x00x00x00x00x00x00x00x15x00x00x01x39x00x00x00x06x00x00x00x00x00x00x00x15x00x00x01x3Ax00x00x00x06x00x00x00x00x00x00x00x15x00x00x01x55x00x00x00x06x00   x00x00x00x00x00x00x15x00x00x01x42x01x00x00x06x00x00x00x00x00x00x00x06x00x00x00x00x00xFFx7F"
$ 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 .
#                                      Default: inspect
#     -a, --arch ARCH                  Specify architecture.
#                                      Supported architectures are .
# 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
# " x00x00x00x04x00x00x00x15x00x00b>x00x00xC0 x00x00x00x00x00x00x005x00x06x00x00x00[email protected]x15x00x04x00x01x00x00x00x15x00x03x00x03x00x00x00x15x00x02x00 x00x00x00x15x00x01x00
# #include 
# #include 
# #include 
#
# 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 "400000000400   0000025000010760000300400000000000000065000600000000100250004000100000025000300030000002500020040000000250001007400000006000000050005000600000000003771770600000000000000"
# _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 .
#     -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
  • Clone the source
    • $ git clone https://github.com/david942j/seccomp-tools && cd seccomp-tools
  • Install dependencies

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




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