Pentesting

Seccomp

Basic Information

Seccomp or Secure Computing mode is a feature of Linux kernel which can act as syscall filter. Seccomp has 2 modes.

Original/Strict Mode

In this mode Seccomp only allow the syscalls exit(), sigreturn(), read() and write() to already-open file descriptors. If any other syscall is made, the process is killed using SIGKILL

seccomp_strict.c
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <linux/seccomp.h>
#include <sys/prctl.h>
//From https://sysdig.com/blog/selinux-seccomp-falco-technical-discussion/
//gcc seccomp_strict.c -o seccomp_strict
int main(int argc, char **argv)
{
int output = open("output.txt", O_WRONLY);
const char *val = "test";
//enables strict seccomp mode
printf("Calling prctl() to set seccomp strict mode...\n");
prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
//This is allowed as the file was already opened
printf("Writing to an already open file...\n");
write(output, val, strlen(val)+1);
//This isn't allowed
printf("Trying to open file for reading...\n");
int input = open("output.txt", O_RDONLY);
printf("You will not see this message--the process will be killed first\n");
}

Seccomp-bpf

This mode allows filtering of system calls using a configurable policy implemented using Berkeley Packet Filter rules.

seccomp_bpf.c
#include <seccomp.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
//https://security.stackexchange.com/questions/168452/how-is-sandboxing-implemented/175373
//gcc seccomp_bpf.c -o seccomp_bpf -lseccomp
void main(void) {
/* initialize the libseccomp context */
scmp_filter_ctx ctx = seccomp_init(SCMP_ACT_KILL);
/* allow exiting */
printf("Adding rule : Allow exit_group\n");
seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(exit_group), 0);
/* allow getting the current pid */
//printf("Adding rule : Allow getpid\n");
//seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(getpid), 0);
printf("Adding rule : Deny getpid\n");
seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EBADF), SCMP_SYS(getpid), 0);
/* allow changing data segment size, as required by glibc */
printf("Adding rule : Allow brk\n");
seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(brk), 0);
/* allow writing up to 512 bytes to fd 1 */
printf("Adding rule : Allow write upto 512 bytes to FD 1\n");
seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 2,
SCMP_A0(SCMP_CMP_EQ, 1),
SCMP_A2(SCMP_CMP_LE, 512));
/* if writing to any other fd, return -EBADF */
printf("Adding rule : Deny write to any FD except 1 \n");
seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EBADF), SCMP_SYS(write), 1,
SCMP_A0(SCMP_CMP_NE, 1));
/* load and enforce the filters */
printf("Load rules and enforce \n");
seccomp_load(ctx);
seccomp_release(ctx);
//Get the getpid is denied, a weird number will be returned like
//this process is -9
printf("this process is %d\n", getpid());
}

Seccomp in Docker

Seccomp-bpf is supported by Docker to restrict the syscalls from the containers effectively decreasing the surface area. You can find the syscalls blocked by default in https://docs.docker.com/engine/security/seccomp/ and the default seccomp profile can be found here https://github.com/moby/moby/blob/master/profiles/seccomp/default.json. You can run a docker container with a different seccomp policy with:

docker run --rm \
-it \
--security-opt seccomp=/path/to/seccomp/profile.json \
hello-world

If you want for example to forbid a container of executing some syscall like uname you could download the default profile from https://github.com/moby/moby/blob/master/profiles/seccomp/default.json and just remove the uname string from the list. If you wan to make sure that some binary doesn't work inside a a docker container you could use strace to list the syscalls the binary is using and then forbid them. In the following example the syscalls of uname are discovered:

ocker run -it --security-opt seccomp=default.json modified-ubuntu strace uname