This script lets you build a Minijail seccomp-bpf filter from strace output. This is very useful if the process that is traced has a fairly tight working domain, and it can be traced in a few scenarios that will exercise all of the needed syscalls. In particular, you should always make sure that failure cases are also exercised to account for calls to abort(2).
If libminijail or minijail0 are used with preloading (the default with dynamically-linked executables), the first few system calls after the first call to execve(2) might not be needed, since the seccomp-bpf filter is installed after that point in a sandboxed process.
strace -f -e raw=all -o strace.txt -- <program> ./tools/generate_seccomp_policy.py strace.txt > <program>.policy
minijail0 may be misattributed to the sandboxed binary and may result in a policy that is overly-permissive. Please pay some extra attention when manually reviewing the allowable args for these syscalls: ioctl, socket, prctl, mmap, mprotect, and mmap2.Linux kernel v4.14+ support SECCOMP_RET_LOG. This allows minijail to log syscalls via the audit subsystem (Redhat has a nice overview here) instead of blocking them. One caveat of this approach is that SECCOMP_RET_LOG does not log syscall arguments for finer grained filtering. The audit subsystem itself has a mechanism to log all syscalls. Though a SYSCALL event is more voluminous than a corresponding SECCOMP event. We employ here a combination of both techniques. We rely on SECCOMP for all except the syscalls for which we want finer grained filtering.
Note that this requires python3 bindings for auparse which are generally available in distro packages named python3-audit or python-audit.
Set up audit rules and an empty seccomp policy for later use. This can be done in the pre-start section of your upstart conf.
$UID is the uid for your process. Using root will lead to logspam.
As mentioned above, these extra audit rules enable SYSCALL auditing which in turn lets the tool inspect arguments for a pre-selected subset of syscalls. The list of syscalls here matches the list of keys in arg_inspection.
for arch in b32 b64; do auditctl -a exit,always -F uid=$UID -F arch=$arch -S ioctl -S socket \ -S prctl -S mmap -S mprotect \ $([ "$arch" = "b32" ] && echo "-S mmap2") -c done touch /tmp/empty.policy
Again, this can be done via your upstart conf. Just be sure to stimulate all corner cases, error conditions, etc for comprehensive coverage.
minijail0 -u $UID -g $GID -L -S /tmp/empty.policy -- <program>
./tools/generate_seccomp_policy.py --audit-comm $PROGRAM_NAME audit.log \ > $PROGRAM_NAME.policy
Note that the tool can also consume multiple audit logs and/or strace traces to produce one unified policy.
An external seccomp-bpf compiler that is documented here. This uses a slightly different syntax and generates highly-optimized BPF binaries that can be provided to minijail0's --seccomp-bpf-binary or libminijail's minijail_set_secomp_filters(). This requires the existence of an architecture-specific constants.json file that contains the mapping of syscall names to numbers, the values of any compile-time constants that could be used to simplify the parameter declaration for filters (like O_RDONLY and any other constant defined in typical headers in /usr/include).
Policy files can also include references to frequency files, which enable profile-guided optimization of the generated BPF code.
The generated BPF code can be analyzed using libseccomp's tools/scmp_bpf_disasm.
make minijail0 constants.json # Create the .policy file using the syntax described in the documentation. cat > test/seccomp.policy <<EOF read: allow write: allow rt_sigreturn: allow exit: allow EOF # Compile the .policy file into a .bpf filter ./tools/compile_seccomp_policy.py test/seccomp.policy test/seccomp.bpf # Load the filter to sandbox your program. ./minijail0 --seccomp-bpf-binary=test/seccomp.bpf -- <program>
This script generates the constants.json file from LLVM IR assembly files. This makes it easier to generate architecture-specific constants.json files at build-time.