easyseccomp

a domain specific language for defining seccomp profiles for containers in an easier way and having more control on the generated BPF that it is possible with libseccomp. This blog post explains more in detail why the project was started: https://www.scrivano.org/posts/2021-01-30-easyseccomp/

A seccomp profile can be defined as:

// Native support for comments without abusing JSON!

#ifdef DENY_MKDIR_WITH_EINVAL
$syscall in (@mkdir) => ERRNO(EINVAL);
#endif

#ifndef DENY_MKDIR_WITH_EINVAL
$syscall in (@mkdir) => ERRNO(EPERM);
#endif

=> ALLOW();

and generate the raw BPF as:

$ easyseccomp < profile.seccomp > seccomp.bpf
$ easyseccomp -d DENY_MKDIR_WITH_EINVAL < profile.seccomp > seccomp.bpf

Language

The policy is a list of CONDITION => STATEMENT; rules that are executed in the specified order. The program terminates performing the action specified STATEMENT for the first CONDITION that is true.

If the CONDITION is not specified (=> STATEMENT();), then the STATEMENT is always performed.

Supported variables

Name	Description
$syscall	The syscall number
$arch	Architecture
$arg0	1st argument to the syscall
$arg1	2nd argument to the syscall
$arg2	3rd argument to the syscall
$arg3	4th argument to the syscall
$arg4	5th argument to the syscall
$arg5	6th argument to the syscall

Actions

Name	Description
ALLOW()	Allow the syscall
TRAP()	Trap the syscall
NOTIFY()	Handle the syscall through a user space handler
LOG()	Log the syscall
KILL()	Kill the process
KILL_PROCESS()	Kill the process
KILL_THREAD()	Kill the thread
ERRNO(ERRNO)	Return the specified error code
TRACE(ERRNO)	Trace the syscall and return the error specified code

Comparison Operators

Name	Description
$variable == VALUE	Equality
$variable != VALUE	Disequality
$variable < VALUE	Less than
$variable <= VALUE	Less than or equal
$variable > VALUE	Greater than
$variable >= VALUE	Greater than or equal
$variable & MASK == VALUE	Bitwise AND
$variable in (SET)	The variable value is part of SET
$variable not in (SET)	The variable value is not part of SET
$syscall in KERNEL(VERSION)	The syscall is part of the specified kernel version

Lookups

When the variable $syscall is used the value can be specified in the form @name and name refers to a syscall name that is looked up using the current architecture.

It is possible to force the lookup for a specific architecture using the format @name@arch

Directives

It is possible to define some rules that are conditionally included in the final BPF:

#ifdef DIRECTIVE_NAME
# ifdef ANOTHER_DIRECTIVE
=> ALLOW();
# endif
#endif

The rules included between the #ifdef and the #endif are included only if both DIRECTIVE_NAME and ANOTHER_DIRECTIVE are specified at compile time.

It enables writing conditional policies such as:

#ifndef CAP_AUDIT_WRITE
$syscall == @socket && $arg0 == 16 && $arg2 == 9 => ERRNO(EINVAL);
#endif

$syscall == @socket => ALLOW();

A higher level tool, such as a container engine, can specify different profiles, In the example above it specifies whether a capability is not added to a container and define a different rule for handling the socket syscall.

Examples

• => ALLOW();: Allow the syscall.
• $syscall in (@read, @write) => ALLOW();: The syscall is one of read or write.
• $syscall not in (4, 5) => ALLOW();: The syscall value is not included in the set (4, 5).
• $syscall == @read && $arg0 == 2 => ALLOW(); The syscall is read and the first argument is 2.
• $syscall ==@write && $arg0 > 2 => ALLOW();: Write to a fd bigger than 2.
• $syscall == @renameat2@aarch64 => ALLOW();: The syscall is value renameat2 as defined for the aarch64 architecture.
• $syscall in KERNEL(5.3): The syscall is present in the kernel 5.3

Dependencies for OCI containers

it currently requires this feature in crun: containers/crun#578

It enables to load a custom raw bpf filter instead of the seccomp configuration specified in the container configuration file.

With that feature in crun, it is possible to create a container using the seccomp profile as:

$ easyseccomp < profile.seccomp > seccomp.bpf
$ podman run --annotation run.oci.seccomp_bpf_file=/tmp/seccomp.bpf --rm fedora mkdir /tmp/foo
mkdir: cannot create directory '/tmp/foo': Operation not permitted

$ easyseccomp DENY_MKDIR_WITH_EINVAL < profile.seccomp > seccomp.bpf
$ podman run --annotation run.oci.seccomp_bpf_file=/tmp/seccomp.bpf --rm fedora mkdir /tmp/foo
mkdir: cannot create directory '/tmp/foo': Invalid argument

BPF generator

easyseccomp uses libseccomp only for the syscall number lookup. It is not used for generating the bpf bytecode as libseccomp internally rewrites the rules.