mirror of
https://github.com/carlospolop/hacktricks
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166 lines
7.3 KiB
Markdown
166 lines
7.3 KiB
Markdown
# Seccomp
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## Basic Information
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**Seccomp** or Secure Computing mode, in summary, is a feature of Linux kernel which can act as **syscall filter**.\
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Seccomp has 2 modes.
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**seccomp** (short for **secure computing mode**) is a computer security facility in the **Linux** **kernel**. seccomp allows a process to make a one-way transition into a "secure" state where **it cannot make any system calls except** `exit()`, `sigreturn()`, `read()` and `write()` to **already-open** file descriptors. Should it attempt any other system calls, the **kernel** will **terminate** the **process** with SIGKILL or SIGSYS. In this sense, it does not virtualize the system's resources but isolates the process from them entirely.
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seccomp mode is **enabled via the `prctl(2)` system call** using the `PR_SET_SECCOMP` argument, or (since Linux kernel 3.17) via the `seccomp(2)` system call. seccomp mode used to be enabled by writing to a file, `/proc/self/seccomp`, but this method was removed in favor of `prctl()`. In some kernel versions, seccomp disables the `RDTSC` x86 instruction, which returns the number of elapsed processor cycles since power-on, used for high-precision timing.
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**seccomp-bpf** is an extension to seccomp that allows **filtering of system calls using a configurable policy** implemented using Berkeley Packet Filter rules. It is used by OpenSSH and vsftpd as well as the Google Chrome/Chromium web browsers on Chrome OS and Linux. (In this regard seccomp-bpf achieves similar functionality, but with more flexibility and higher performance, to the older systrace—which seems to be no longer supported for Linux.)
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### **Original/Strict Mode**
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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
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{% code title="seccomp_strict.c" %}
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```c
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#include <fcntl.h>
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#include <stdio.h>
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#include <unistd.h>
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#include <string.h>
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#include <linux/seccomp.h>
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#include <sys/prctl.h>
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//From https://sysdig.com/blog/selinux-seccomp-falco-technical-discussion/
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//gcc seccomp_strict.c -o seccomp_strict
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int main(int argc, char **argv)
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{
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int output = open("output.txt", O_WRONLY);
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const char *val = "test";
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//enables strict seccomp mode
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printf("Calling prctl() to set seccomp strict mode...\n");
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prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
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//This is allowed as the file was already opened
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printf("Writing to an already open file...\n");
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write(output, val, strlen(val)+1);
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//This isn't allowed
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printf("Trying to open file for reading...\n");
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int input = open("output.txt", O_RDONLY);
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printf("You will not see this message--the process will be killed first\n");
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}
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```
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{% endcode %}
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### Seccomp-bpf
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This mode allows f**iltering of system calls using a configurable policy** implemented using Berkeley Packet Filter rules.
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{% code title="seccomp_bpf.c" %}
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```c
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#include <seccomp.h>
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#include <unistd.h>
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#include <stdio.h>
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#include <errno.h>
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//https://security.stackexchange.com/questions/168452/how-is-sandboxing-implemented/175373
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//gcc seccomp_bpf.c -o seccomp_bpf -lseccomp
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void main(void) {
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/* initialize the libseccomp context */
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scmp_filter_ctx ctx = seccomp_init(SCMP_ACT_KILL);
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/* allow exiting */
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printf("Adding rule : Allow exit_group\n");
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seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(exit_group), 0);
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/* allow getting the current pid */
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//printf("Adding rule : Allow getpid\n");
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//seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(getpid), 0);
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printf("Adding rule : Deny getpid\n");
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seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EBADF), SCMP_SYS(getpid), 0);
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/* allow changing data segment size, as required by glibc */
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printf("Adding rule : Allow brk\n");
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seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(brk), 0);
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/* allow writing up to 512 bytes to fd 1 */
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printf("Adding rule : Allow write upto 512 bytes to FD 1\n");
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seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 2,
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SCMP_A0(SCMP_CMP_EQ, 1),
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SCMP_A2(SCMP_CMP_LE, 512));
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/* if writing to any other fd, return -EBADF */
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printf("Adding rule : Deny write to any FD except 1 \n");
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seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EBADF), SCMP_SYS(write), 1,
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SCMP_A0(SCMP_CMP_NE, 1));
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/* load and enforce the filters */
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printf("Load rules and enforce \n");
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seccomp_load(ctx);
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seccomp_release(ctx);
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//Get the getpid is denied, a weird number will be returned like
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//this process is -9
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printf("this process is %d\n", getpid());
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}
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```
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{% endcode %}
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## Seccomp in Docker
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**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/](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](https://github.com/moby/moby/blob/master/profiles/seccomp/default.json).\
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You can run a docker container with a **different seccomp** policy with:
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```bash
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docker run --rm \
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-it \
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--security-opt seccomp=/path/to/seccomp/profile.json \
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hello-world
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```
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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](https://github.com/moby/moby/blob/master/profiles/seccomp/default.json) and just **remove the `uname` string from the list**.\
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If you want 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.\
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In the following example the **syscalls** of `uname` are discovered:
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```bash
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docker run -it --security-opt seccomp=default.json modified-ubuntu strace uname
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```
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{% hint style="info" %}
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If you are using **Docker just to launch an application**, you can **profile** it with **`strace`** and **just allow the syscalls** it needs
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{% endhint %}
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### Example Seccomp policy
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To illustrate Seccomp feature, let’s create a Seccomp profile disabling “chmod” system call as below.
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```json
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{
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"defaultAction": "SCMP_ACT_ALLOW",
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"syscalls": [
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{
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"name": "chmod",
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"action": "SCMP_ACT_ERRNO"
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}
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]
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}
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```
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In the above profile, we have set default action to “allow” and created a black list to disable “chmod”. To be more secure, we can set default action to drop and create a white list to selectively enable system calls.\
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Following output shows the “chmod” call returning error because its disabled in the seccomp profile
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```bash
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$ docker run --rm -it --security-opt seccomp:/home/smakam14/seccomp/profile.json busybox chmod 400 /etc/hosts
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chmod: /etc/hosts: Operation not permitted
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```
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Following output shows the “docker inspect” displaying the profile:
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```json
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"SecurityOpt": [
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"seccomp:{\"defaultAction\":\"SCMP_ACT_ALLOW\",\"syscalls\":[{\"name\":\"chmod\",\"action\":\"SCMP_ACT_ERRNO\"}]}"
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],
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```
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### Deactivate it in Docker
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Launch a container with the flag: **`--security-opt seccomp=unconfined`**
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Also, note that when Docker (or other CRIs) are used in a **Kubernetes** cluster, the **seccomp filter is disabled by default**
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