WHAT IS IT?

Kafel is a language and library for specifying syscall filtering policies. The policies are compiled into BPF code that can be used with seccomp-filter.

This is NOT an official Google product.

Usage

With verbose error reporting

struct sock_fprog prog;
kafel_ctxt_t ctxt = kafel_ctxt_create();
kafel_set_input_string(ctxt, seccomp_policy);
if (kafel_compile(ctxt, &prog)) {
  fprintf(stderr, "policy compilation failed: %s", kafel_error_msg(ctxt));
  kafel_ctxt_destroy(&ctxt);
  exit(-1);
}
kafel_ctxt_destroy(&ctxt);
prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog, 0, 0);
free(prog.filter);

Without verbose error reporting

struct sock_fprog prog;
if (kafel_compile_string(seccomp_policy, &prog)) {
  fputs("policy compilation failed", stderr);
  exit(-1);
}
prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog, 0, 0);
free(prog.filter);

Policy language

A simple language is used to define policies.

A policy file has 3 parts:

1. Constant definitions (optional)
2. Policy definitions
3. Top level policy declaration

Numbers

Kafel supports following number notations:

• Decimal 42
• Hexadecimal 0xfa1
• Octal 0777
• Binary 0b10101

Constant definitions

You may define numeric constants at the beging of policy file to make it more readable. The defined constants can then be used anywhere where a number is expected.

#define MYCONST 123

Policy definitions

Policy definition is a list of action blocks and use statements separated by commas.

samples/ contains some example policies that demonstrate supported features.

Use statements

A USE someOtherPolicy behaves as if someOtherPolicy body was pasted in its place. You may only use policies defined before the use statement.

With use statements you can create meaningful groups of filtering rules that are building blocks of bigger policies.

Action blocks

Action block consist of a target and list of syscall matching rules separated with commas.

Target of first rule matched is the policy decision.

Following table list Kafel targets and their corresponding seccomp-filter return values.

Kafel	seccomp-filter
ALLOW	SECCOMP_RET_ALLOW
KILL, DENY	SECCOMP_RET_KILL
ERRNO(number)	SECCOMP_RET_ERRNO+number
TRAP(number)	SECCOMP_RET_TRAP+number
TRACE(number)	SECCOMP_RET_TRACE+number

Syscall matching rules

A rules consist of syscall name and optional list of boolean expressions.

List of boolean expressions separated by commas. A comma is semantically equivalent to || but has the lowest precedence, therefore it may be easier to read.

Syscall naming

Normally syscalls are specified by their names as defined in Linux kernel. However, you may also filter custom syscalls that are not in the standard syscall list. You can either define a constant and use it in place of syscall name or utilize SYSCALL keyword.

#define mysyscall -1

POLICY my_const {
  ALLOW {
    mysyscall
  }
}

POLICY my_literal {
  ALLOW {
    SYSCALL[-1]
  }
}

Argument filtering

Boolean expressions are used to filter syscalls based on their arguments. A expression resembles C language syntax, except that there are no arithmetic operators.

some_syscall(first_arg, my_arg_name) { first_arg == 42 && my_arg_name != 42 }

Bitwise and (&) operator can be used to test for flags.

mmap { (prot & PROT_EXEC) == 0 }

You don't have to declare arguments for well-known syscalls but can just use their regular names as specified in Linux kernel and man pages.

write { fd == 1 }

Top level policy declaration

USE topLevel DEFAULT the_action

Specifies that topLevel policy is compiled and action the_action should be taken when no rule matches.

#Example

When used with nsjail, the following command allows to create a fairly constrained environment for your shell

$ ./nsjail --chroot / --seccomp_string 'POLICY a { ALLOW { write, execve, brk, access, mmap, open, newfstat, close, read, mprotect, arch_prctl, munmap, getuid, getgid, getpid, rt_sigaction, geteuid, getppid, getcwd, getegid, ioctl, fcntl, newstat, clone, wait4, rt_sigreturn, exit_group } } USE a DEFAULT KILL' -- /bin/sh -i

[2017-01-15T21:53:08+0100] Mode: STANDALONE_ONCE
[2017-01-15T21:53:08+0100] Jail parameters: hostname:'NSJAIL', chroot:'/', process:'/bin/sh', bind:[::]:0, max_conns_per_ip:0, uid:(ns:1000, global:1000), gid:(ns:1000, global:1000), time_limit:0, personality:0, daemonize:false, clone_newnet:true, clone_newuser:true, clone_newns:true, clone_newpid:true, clone_newipc:true, clonew_newuts:true, clone_newcgroup:false, keep_caps:false, tmpfs_size:4194304, disable_no_new_privs:false, pivot_root_only:false
[2017-01-15T21:53:08+0100] Mount point: src:'/' dst:'/' type:'' flags:0x5001 options:''
[2017-01-15T21:53:08+0100] Mount point: src:'(null)' dst:'/proc' type:'proc' flags:0x0 options:''
[2017-01-15T21:53:08+0100] PID: 18873 about to execute '/bin/sh' for [STANDALONE_MODE]
/bin/sh: 0: can't access tty; job control turned off
$ set
IFS='
'
OPTIND='1'
PATH='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
PPID='0'
PS1='$ '
PS2='> '
PS4='+ '
PWD='/'
$ id
Bad system call
$ exit
[2017-01-15T21:53:17+0100] PID: 18873 exited with status: 159, (PIDs left: 0)