seccomp

In the previous page, we explored the concepts and strategies behind system call filtering. Now we dive deep into seccomp (secure computing mode)—Linux's kernel facility for implementing these filters.

Seccomp represents one of the most important security innovations in the Linux kernel. Introduced progressively between 2005 and 2012, it has become the foundation of sandboxing in Chrome, Docker, Android, systemd, and countless other security-critical systems. Understanding seccomp is essential for anyone building or analyzing sandboxed systems on Linux.

Seccomp operates at the system call boundary, making decisions about whether to allow, deny, or otherwise handle each syscall before it executes. Because it runs in kernel context, seccomp provides guarantees that user-space monitoring cannot: there is no race condition between the check and the syscall execution.

Seccomp operates at a critical point in the kernel: the system call entry path. When a user-space process invokes a system call, the kernel's syscall handler checks if seccomp filters are installed before executing the actual syscall handler.

Execution Flow:

User Space                    Kernel Space
─────────────────────────────────────────────────────
                              
process → syscall →   ──────► syscall entry
                              │
                              ▼
                              seccomp filter check
                              │
                    ┌─────────┴──────────┐
                    │                    │
                    ▼                    ▼
             ALLOW ───► syscall      KILL ───► terminate
                        handler           │
                        │                 ▼
                        ▼              ERRNO ───► return error
                    ◄── return        TRAP ────► SIGSYS signal
                                      NOTIFY ──► supervisor

Key Architectural Properties:

1. Filter Runs in Kernel Context: The BPF filter executes in kernel space during syscall entry. This is critical—user space cannot race with or manipulate the filter execution.

2. Filters Are Inherited: Child processes inherit their parent's seccomp filters. Once a filter is installed, it applies to all current and future threads.

3. Filters Are Append-Only: You can add more restrictive filters but cannot remove filters. This prevents an attacker who gains code execution from disabling the sandbox.

4. Filters Stack: Multiple filters can be installed. The kernel evaluates all filters, and the most restrictive result wins (lowest precedence value).

Seccomp has two distinct modes of operation, reflecting its evolution over time:

Mode 1: Strict Mode (Original seccomp)

The original seccomp (2005) provides an extremely simple, fixed policy: only four syscalls are allowed: read, write, exit, and sigreturn. Any other syscall immediately terminates the process.

#include <linux/seccomp.h>
#include <sys/prctl.h>

void enable_strict_seccomp() {
    // Enable strict seccomp mode
    if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT) != 0) {
        perror("prctl");
        exit(1);
    }
    
    // From here, only read/write/exit/sigreturn work
    // Any other syscall = instant death
}

Use Cases for Strict Mode:

• Extremely simple compute tasks (no file opening, no memory allocation after setup)
• CPUjail-style sandboxes for untrusted computation
• Historical interest—mostly superseded by filter mode

Limitations:

• Too restrictive for most applications (can't even allocate memory)
• No customization—fixed policy
• Can't communicate except through pre-opened file descriptors

Mode 2: Filter Mode (seccomp-bpf)

Filter mode (2012) extends seccomp with BPF (Berkeley Packet Filter) programs, allowing flexible, programmable policies. The filter can inspect the syscall number, arguments, and architecture, returning any of the action codes.

#include <linux/seccomp.h>
#include <linux/filter.h>
#include <sys/prctl.h>

void enable_seccomp_filter(struct sock_fprog *prog) {
    // Required: prevent privilege escalation through exec of setuid binaries
    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) != 0) {
        perror("prctl NO_NEW_PRIVS");
        exit(1);
    }
    
    // Install seccomp filter
    if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog) != 0) {
        perror("prctl SECCOMP_MODE_FILTER");
        exit(1);
    }
}

// Alternative: use seccomp() syscall directly
void enable_seccomp_syscall(struct sock_fprog *prog): {
    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) != 0) {
        perror("prctl NO_NEW_PRIVS");
        exit(1);
    }
    
    if (seccomp(SECCOMP_SET_MODE_FILTER, 0, prog) != 0) {
        perror("seccomp");
        exit(1);
    }
}

Seccomp uses classic BPF (cBPF) for filter programs—a simple bytecode virtual machine originally designed for packet filtering. A BPF program consists of instructions that load values, perform arithmetic, and make conditional jumps.

The seccomp_data Structure:

The BPF filter can access syscall information through the seccomp_data structure:

struct seccomp_data {
    int   nr;         // Syscall number
    __u32 arch;       // AUDIT_ARCH_ value (architecture)
    __u64 instruction_pointer;  // Return address
    __u64 args[6];    // Syscall arguments
};

BPF Instructions:

BPF uses a small instruction set:

Example: Minimal Allowlist Filter:

#include <linux/seccomp.h>
#include <linux/filter.h>
#include <linux/audit.h>
#include <sys/syscall.h>

struct sock_filter filter[] = {
    // Load architecture
    BPF_STMT(BPF_LD | BPF_W | BPF_ABS, 
             offsetof(struct seccomp_data, arch)),
    // Verify architecture is x86_64
    BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, AUDIT_ARCH_X86_64, 1, 0),
    // Kill if wrong architecture
    BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),
    
    // Load syscall number
    BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
             offsetof(struct seccomp_data, nr)),
    
    // Allow read (syscall 0)
    BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_read, 0, 1),
    BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
    
    // Allow write (syscall 1)
    BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_write, 0, 1),
    BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
    
    // Allow exit_group (syscall 231)
    BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_exit_group, 0, 1),
    BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
    
    // Default: kill
    BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),
};

struct sock_fprog prog = {
    .len = sizeof(filter) / sizeof(filter[0]),
    .filter = filter,
};

Understanding the BPF_STMT and BPF_JUMP Macros:

// BPF_STMT: unconditional instruction
// BPF_STMT(code, k)
// code = operation type | size | mode
// k = immediate value

BPF_STMT(BPF_LD | BPF_W | BPF_ABS, offsetof(struct seccomp_data, nr))
// BPF_LD: load operation
// BPF_W: word (32-bit) size
// BPF_ABS: absolute addressing from filter data (seccomp_data)
// Result: load seccomp_data.nr into accumulator

// BPF_JUMP: conditional jump
// BPF_JUMP(code, k, jt, jf)
// code = operation type | comparison
// k = comparison value
// jt = instructions to skip if true
// jf = instructions to skip if false

BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_read, 0, 1)
// BPF_JEQ: jump if equal
// BPF_K: compare with immediate value k
// If accumulator == __NR_read: skip 0 instructions (execute next)
// If accumulator != __NR_read: skip 1 instruction

libseccomp provides a high-level, architecture-independent API for creating seccomp filters. It handles the complexities of BPF generation, architecture differences, and syscall number translation.

Basic Usage:

#include <seccomp.h>

void setup_seccomp() {
    // Create filter context with default action KILL_PROCESS
    scmp_filter_ctx ctx = seccomp_init(SCMP_ACT_KILL_PROCESS);
    if (ctx == NULL) {
        perror("seccomp_init");
        exit(1);
    }
    
    // Allow specific syscalls
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(read), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(close), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fstat), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mmap), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mprotect), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(munmap), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(brk), 0);
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(exit_group), 0);
    
    // Load filter into kernel
    if (seccomp_load(ctx) != 0) {
        perror("seccomp_load");
        exit(1);
    }
    
    // Release context (filter is now in kernel)
    seccomp_release(ctx);
}

Argument Filtering with libseccomp:

libseccomp makes argument filtering straightforward:

#include <seccomp.h>

void setup_with_arg_filtering() {
    scmp_filter_ctx ctx = seccomp_init(SCMP_ACT_KILL);
    
    // Allow socket() only for AF_UNIX (argument 0 == 1)
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(socket), 1,
                     SCMP_A0(SCMP_CMP_EQ, AF_UNIX));
    
    // Allow mprotect() but not with PROT_EXEC | PROT_WRITE
    // (Block arg2 having both bits set)
    seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(mprotect), 1,
                     SCMP_A2(SCMP_CMP_MASKED_EQ, 
                            PROT_EXEC | PROT_WRITE, 
                            PROT_EXEC | PROT_WRITE));
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mprotect), 0);
    
    // Allow ioctl() only for specific commands
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(ioctl), 1,
                     SCMP_A1(SCMP_CMP_EQ, TCGETS));
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(ioctl), 1,
                     SCMP_A1(SCMP_CMP_EQ, FIONREAD));
    
    // Block openat with O_CREAT (prevent file creation)
    seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(openat), 1,
                     SCMP_A2(SCMP_CMP_MASKED_EQ, O_CREAT, O_CREAT));
    seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(openat), 0);
    
    seccomp_load(ctx);
    seccomp_release(ctx);
}

Comparison Operators:

Architecture Handling:

libseccomp automatically handles architecture differences:

// Enable support for additional architectures
seccomp_arch_add(ctx, SCMP_ARCH_X86);      // 32-bit x86
seccomp_arch_add(ctx, SCMP_ARCH_X32);      // x32 ABI

// Or remove architectures (stricter)
seccomp_arch_remove(ctx, SCMP_ARCH_NATIVE); // Remove native first
seccomp_arch_add(ctx, SCMP_ARCH_X86_64);    // Add only x86_64
// Now 32-bit and x32 syscalls will be blocked

SECCOMP_RET_USER_NOTIF (Linux 5.0+) enables a powerful broker pattern where a supervisor process handles blocked syscalls on behalf of the sandboxed process. The supervisor can inspect the syscall, validate it against policy, and either perform the operation or deny it.

Architecture:

┌─────────────────┐                    ┌─────────────────┐
│  Sandboxed      │                    │   Supervisor    │
│  Process        │                    │   Process       │
├─────────────────┤                    ├─────────────────┤
│                 │                    │                 │
│  syscall(open)  │──► blocked by ─────│  notif_recv()  │
│      ▼          │    seccomp         │      ▼          │
│   [blocked]     │                    │  validate path  │
│                 │                    │      ▼          │
│                 │                    │  open() if OK   │
│                 │                    │      ▼          │
│  [resumes]  ◄───│◄─ notif_send() ────│  send fd back   │
│   with fd       │                    │                 │
└─────────────────┘                    └─────────────────┘

Setting Up User Notification:

#include <linux/seccomp.h>
#include <sys/ioctl.h>

int setup_notify_supervisor() {
    struct sock_filter filter[] = {
        // ... architecture check ...
        
        // Load syscall number
        BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                 offsetof(struct seccomp_data, nr)),
        
        // Send openat to supervisor
        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_openat, 0, 1),
        BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_USER_NOTIF),
        
        // Allow other syscalls
        BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
    };
    
    struct sock_fprog prog = {
        .len = sizeof(filter) / sizeof(filter[0]),
        .filter = filter,
    };
    
    // Install filter and get notification fd
    int notify_fd = seccomp(SECCOMP_SET_MODE_FILTER,
                            SECCOMP_FILTER_FLAG_NEW_LISTENER,
                            &prog);
    if (notify_fd < 0) {
        perror("seccomp");
        exit(1);
    }
    
    return notify_fd;  // Parent uses this fd to receive notifications
}

Supervisor Event Loop:

void supervisor_loop(int notify_fd) {
    struct seccomp_notif *req = NULL;
    struct seccomp_notif_resp *resp = NULL;
    struct seccomp_notif_sizes sizes;
    
    // Get sizes for allocation
    seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes);
    req = malloc(sizes.seccomp_notif);
    resp = malloc(sizes.seccomp_notif_resp);
    
    while (1) {
        memset(req, 0, sizes.seccomp_notif);
        memset(resp, 0, sizes.seccomp_notif_resp);
        
        // Receive notification (blocks until syscall intercepted)
        if (ioctl(notify_fd, SECCOMP_IOCTL_NOTIF_RECV, req) != 0) {
            if (errno == ENOENT)  // Target died
                continue;
            perror("NOTIF_RECV");
            break;
        }
        
        // req->id: unique notification ID
        // req->pid: pid of sandboxed process
        // req->data.nr: syscall number
        // req->data.args[]: syscall arguments
        
        resp->id = req->id;
        resp->flags = 0;
        
        if (req->data.nr == __NR_openat) {
            // Read pathname from sandboxed process memory
            char path[PATH_MAX];
            if (read_process_memory(req->pid, req->data.args[1], 
                                    path, sizeof(path)) < 0) {
                resp->error = -EACCES;
            } else if (is_path_allowed(path)) {
                // Perform open on behalf of sandbox
                int fd = openat(req->data.args[0], path,
                               req->data.args[2], req->data.args[3]);
                if (fd >= 0) {
                    // Send fd to sandboxed process
                    resp->val = fd;
                    resp->flags = SECCOMP_USER_NOTIF_FLAG_CONTINUE;
                    // Actually, for fd injection, use SECCOMP_IOCTL_NOTIF_ADDFD
                } else {
                    resp->error = -errno;
                }
            } else {
                resp->error = -EACCES;
            }
        }
        
        // Send response (unblocks sandboxed process)
        if (ioctl(notify_fd, SECCOMP_IOCTL_NOTIF_SEND, resp) != 0) {
            perror("NOTIF_SEND");
        }
    }
}

The seccomp() system call accepts various flags that modify filter behavior:

SECCOMP_FILTER_FLAG_TSYNC:

Synchronize the filter across all threads in the thread group. Without this, each thread needs to install the filter individually:

prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_TSYNC, &prog);
// Now all existing threads have the filter

SECCOMP_FILTER_FLAG_LOG:

Log all filtered syscalls that match a LOG action. Also ensures non-LOG actions are logged if audit is enabled:

seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_LOG, &prog);
// dmesg shows: seccomp filter LOG action

SECCOMP_FILTER_FLAG_SPEC_ALLOW:

Disable Spectre mitigations for this filter. Can improve performance but reduces security against Spectre attacks:

// Only use if performance is critical and you understand Spectre risks
seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_SPEC_ALLOW, &prog);

seccomp Attribute Operations:

Recent kernels support querying and setting seccomp attributes:

struct seccomp_notif_sizes sizes;

// Get required allocation sizes for notification structures
seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes);
printf("notif: %hu, notif_resp: %hu, data: %hu
",
       sizes.seccomp_notif, sizes.seccomp_notif_resp, 
       sizes.seccomp_data);

// Get current seccomp action (for diagnostic)
int action;
seccomp(SECCOMP_GET_ACTION_AVAIL, 0, &action);

Production seccomp implementations follow established patterns that balance security with reliability:

Pattern: Privileged Setup, Then Sandbox:

Perform all privileged operations before installing the filter:

int main() {
    // Phase 1: Privileged setup
    int listen_fd = socket(AF_INET, SOCK_STREAM, 0);
    bind(listen_fd, ...);  // Bind to port 80
    listen(listen_fd, 100);
    
    // Open config files, allocate resources, etc.
    config_t *cfg = load_config("/etc/myservice.conf");
    
    // Phase 2: Drop privileges
    drop_root_privileges();
    
    // Phase 3: Install seccomp filter
    install_seccomp_filter();
    
    // Phase 4: Run main loop (now sandboxed)
    while (1) {
        int conn = accept(listen_fd, ...);
        handle_connection(conn);  // Cannot open files, bind ports, etc.
    }
}

Pattern: Fail-Open Development, Fail-Closed Production:

void install_filter(int strict_mode) {
    scmp_filter_ctx ctx = seccomp_init(
        strict_mode ? SCMP_ACT_KILL : SCMP_ACT_LOG
    );
    // ... rules ...
    seccomp_load(ctx);
}

// Development: blocked syscalls are logged, not killed
install_filter(false);
// Production: blocked syscalls terminate process
install_filter(true);

Pattern: Sandbox Entry Function:

int enter_sandbox(void (*sandboxed_main)(void *), void *arg) {
    // 1. Validate we're in a clean state
    if (getuid() == 0) {
        fprintf(stderr, "Must not be root
");
        return -1;
    }
    
    // 2. Set up additional isolation
    if (unshare(CLONE_NEWNET | CLONE_NEWNS | CLONE_NEWIPC) != 0) {
        perror("unshare");
        // Continue anyway—seccomp is the primary protection
    }
    
    // 3. Set NO_NEW_PRIVS
    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) != 0) {
        perror("PR_SET_NO_NEW_PRIVS");
        return -1;
    }
    
    // 4. Drop capabilities
    drop_all_capabilities();
    
    // 5. Install seccomp filter
    if (install_seccomp_filter() != 0) {
        return -1;
    }
    
    // 6. Close unnecessary file descriptors
    close_fds_above(2);
    
    // 7. Call sandboxed code
    sandboxed_main(arg);
    
    return 0;
}