Seccomp security profiles for Docker

Estimated reading time: 8 minutes

Secure computing mode (Seccomp) is a Linux kernel feature. You can use it to restrict the actions available within the container. The seccomp() system call operates on the seccomp state of the calling process. You can use this feature to restrict your application’s access.

This feature is available only if Docker has been built with seccomp and the kernel is configured with CONFIG_SECCOMP enabled. To check if your kernel supports seccomp:

$ cat /boot/config-`uname -r` | grep CONFIG_SECCOMP=
CONFIG_SECCOMP=y

Note: seccomp profiles require seccomp 2.2.1 and are only available starting with Debian 9 “Stretch”, Ubuntu 16.04 “Xenial”, Fedora 22, CentOS 7 and Oracle Linux 7. To use this feature on Ubuntu 14.04, Debian Wheezy, or Debian Jessie, you must download the latest static Docker Linux binary. This feature is currently not available on other distributions.

Passing a profile for a container

The default seccomp profile provides a sane default for running containers with seccomp and disables around 44 system calls out of 300+. It is moderately protective while providing wide application compatibility. The default Docker profile has a JSON layout in the following form:

{
	"defaultAction": "SCMP_ACT_ERRNO",
	"archMap": [
		{
			"architecture": "SCMP_ARCH_X86_64",
			"subArchitectures": [
				"SCMP_ARCH_X86",
				"SCMP_ARCH_X32"
			]
		},
		...
	],
	"syscalls": [
		{
			"names": [
				"accept",
				"accept4",
				"access",
				"alarm",
				"alarm",
				"bind",
				"brk",
				...
				"waitid",
				"waitpid",
				"write",
				"writev"
			],
			"action": "SCMP_ACT_ALLOW",
			"args": [],
			"comment": "",
			"includes": {},
			"excludes": {}
		},
		{
			"names": [
				"clone"
			],
			"action": "SCMP_ACT_ALLOW",
			"args": [
				{
					"index": 1,
					"value": 2080505856,
					"valueTwo": 0,
					"op": "SCMP_CMP_MASKED_EQ"
				}
			],
			"comment": "s390 parameter ordering for clone is different",
			"includes": {
				"arches": [
					"s390",
					"s390x"
				]
			},
			"excludes": {
				"caps": [
					"CAP_SYS_ADMIN"
				]
			}
		},
		...
}

When you run a container, it uses the default profile unless you override it with the security-opt option. For example, the following explicitly specifies the default policy:

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

Significant syscalls blocked by the default profile

Docker’s default seccomp profile is a whitelist which specifies the calls that are allowed. The table below lists the significant (but not all) syscalls that are effectively blocked because they are not on the whitelist. The table includes the reason each syscall is blocked rather than white-listed.

Syscall Description
acct Accounting syscall which could let containers disable their own resource limits or process accounting. Also gated by CAP_SYS_PACCT.
add_key Prevent containers from using the kernel keyring, which is not namespaced.
adjtimex Similar to clock_settime and settimeofday, time/date is not namespaced. Also gated by CAP_SYS_TIME
bpf Deny loading potentially persistent bpf programs into kernel, already gated by CAP_SYS_ADMIN.
clock_adjtime Time/date is not namespaced. Also gated by CAP_SYS_TIME.
clock_settime Time/date is not namespaced. Also gated by CAP_SYS_TIME.
clone Deny cloning new namespaces. Also gated by CAP_SYS_ADMIN for CLONE_* flags, except CLONE_USERNS.
create_module Deny manipulation and functions on kernel modules. Obsolete. Also gated by CAP_SYS_MODULE
delete_module Deny manipulation and functions on kernel modules. Also gated by CAP_SYS_MODULE.
finit_module Deny manipulation and functions on kernel modules. Also gated by CAP_SYS_MODULE.
get_kernel_syms Deny retrieval of exported kernel and module symbols. Obsolete.
get_mempolicy Syscall that modifies kernel memory and NUMA settings. Already gated by CAP_SYS_NICE.
init_module Deny manipulation and functions on kernel modules. Also gated by CAP_SYS_MODULE.
ioperm Prevent containers from modifying kernel I/O privilege levels. Already gated by CAP_SYS_RAWIO.
iopl Prevent containers from modifying kernel I/O privilege levels. Already gated by CAP_SYS_RAWIO.
kcmp Restrict process inspection capabilities, already blocked by dropping CAP_PTRACE.
kexec_file_load Sister syscall of kexec_load that does the same thing, slightly different arguments. Also gated by CAP_SYS_BOOT.
kexec_load Deny loading a new kernel for later execution. Also gated by CAP_SYS_BOOT.
keyctl Prevent containers from using the kernel keyring, which is not namespaced.
lookup_dcookie Tracing/profiling syscall, which could leak a lot of information on the host. Also gated by CAP_SYS_ADMIN.
mbind Syscall that modifies kernel memory and NUMA settings. Already gated by CAP_SYS_NICE.
mount Deny mounting, already gated by CAP_SYS_ADMIN.
move_pages Syscall that modifies kernel memory and NUMA settings.
name_to_handle_at Sister syscall to open_by_handle_at. Already gated by CAP_SYS_NICE.
nfsservctl Deny interaction with the kernel nfs daemon. Obsolete since Linux 3.1.
open_by_handle_at Cause of an old container breakout. Also gated by CAP_DAC_READ_SEARCH.
perf_event_open Tracing/profiling syscall, which could leak a lot of information on the host.
personality Prevent container from enabling BSD emulation. Not inherently dangerous, but poorly tested, potential for a lot of kernel vulns.
pivot_root Deny pivot_root, should be privileged operation.
process_vm_readv Restrict process inspection capabilities, already blocked by dropping CAP_PTRACE.
process_vm_writev Restrict process inspection capabilities, already blocked by dropping CAP_PTRACE.
ptrace Tracing/profiling syscall, which could leak a lot of information on the host. Already blocked by dropping CAP_PTRACE.
query_module Deny manipulation and functions on kernel modules. Obsolete.
quotactl Quota syscall which could let containers disable their own resource limits or process accounting. Also gated by CAP_SYS_ADMIN.
reboot Don’t let containers reboot the host. Also gated by CAP_SYS_BOOT.
request_key Prevent containers from using the kernel keyring, which is not namespaced.
set_mempolicy Syscall that modifies kernel memory and NUMA settings. Already gated by CAP_SYS_NICE.
setns Deny associating a thread with a namespace. Also gated by CAP_SYS_ADMIN.
settimeofday Time/date is not namespaced. Also gated by CAP_SYS_TIME.
stime Time/date is not namespaced. Also gated by CAP_SYS_TIME.
swapon Deny start/stop swapping to file/device. Also gated by CAP_SYS_ADMIN.
swapoff Deny start/stop swapping to file/device. Also gated by CAP_SYS_ADMIN.
sysfs Obsolete syscall.
_sysctl Obsolete, replaced by /proc/sys.
umount Should be a privileged operation. Also gated by CAP_SYS_ADMIN.
umount2 Should be a privileged operation. Also gated by CAP_SYS_ADMIN.
unshare Deny cloning new namespaces for processes. Also gated by CAP_SYS_ADMIN, with the exception of unshare --user.
uselib Older syscall related to shared libraries, unused for a long time.
userfaultfd Userspace page fault handling, largely needed for process migration.
ustat Obsolete syscall.
vm86 In kernel x86 real mode virtual machine. Also gated by CAP_SYS_ADMIN.
vm86old In kernel x86 real mode virtual machine. Also gated by CAP_SYS_ADMIN.

Run without the default seccomp profile

You can pass unconfined to run a container without the default seccomp profile.

$ docker run --rm -it --security-opt seccomp=unconfined debian:jessie \
    unshare --map-root-user --user sh -c whoami
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