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# Seccomp ## Basic Information **Seccomp** or Secure Computing mode, in summary, is a feature of Linux kernel which can act as **syscall filter**.\ Seccomp has 2 modes. **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. 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. **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.) ### **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 {% code title="seccomp_strict.c" %} ```c #include #include #include #include #include #include //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"); } ``` {% endcode %} ### Seccomp-bpf This mode allows f**iltering of system calls using a configurable policy** implemented using Berkeley Packet Filter rules. {% code title="seccomp_bpf.c" %} ```c #include #include #include #include //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()); } ``` {% endcode %} ## 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/](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).\ You can run a docker container with a **different seccomp** policy with: ```bash 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](https://github.com/moby/moby/blob/master/profiles/seccomp/default.json) and just **remove the `uname` string from the list**.\ 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.\ In the following example the **syscalls** of `uname` are discovered: ```bash docker run -it --security-opt seccomp=default.json modified-ubuntu strace uname ``` {% hint style="info" %} If you are using **Docker just to launch an application**, you can **profile** it with **`strace`** and **just allow the syscalls** it needs {% endhint %} ### Example Seccomp policy To illustrate Seccomp feature, let’s create a Seccomp profile disabling “chmod” system call as below. ```json { "defaultAction": "SCMP_ACT_ALLOW", "syscalls": [ { "name": "chmod", "action": "SCMP_ACT_ERRNO" } ] } ``` 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.\ Following output shows the “chmod” call returning error because its disabled in the seccomp profile ```bash $ docker run --rm -it --security-opt seccomp:/home/smakam14/seccomp/profile.json busybox chmod 400 /etc/hosts chmod: /etc/hosts: Operation not permitted ``` Following output shows the “docker inspect” displaying the profile: ```json "SecurityOpt": [ "seccomp:{\"defaultAction\":\"SCMP_ACT_ALLOW\",\"syscalls\":[{\"name\":\"chmod\",\"action\":\"SCMP_ACT_ERRNO\"}]}" ], ``` ### Deactivate it in Docker Launch a container with the flag: **`--security-opt seccomp=unconfined`** 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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