Proof-of-concept, default-permit seccomp-bpf sandbox

Can be disabled by adding -DCONFIG_NOSANDBOX to CXXFLAGS.

src/sandbox/seccomp.cpp

@@ -0,0 +1,102 @@
+#if defined(__linux__) && !defined(CONFIG_NOSANDBOX)
+
+#include "core/Core.hpp" // mostly for ARRLEN
+#include "settings.hpp"
+
+#include <stdlib.h>
+
+#include <sys/prctl.h>
+#include <sys/ptrace.h>
+
+#include <linux/unistd.h>
+#include <linux/seccomp.h>
+#include <linux/filter.h>
+#include <linux/audit.h>
+
+// our own wrapper for the seccomp() syscall
+// TODO: should this be conditional on a feature check or something?
+static inline int seccomp(unsigned int operation, unsigned int flags, void *args) {
+    return syscall(__NR_seccomp, operation, flags, args);
+}
+
+/*
+ * Macros borrowed from from https://outflux.net/teach-seccomp/
+ * Relevant license:
+ *     https://source.chromium.org/chromium/chromium/src/+/master:LICENSE
+ */
+#define syscall_nr (offsetof(struct seccomp_data, nr))
+#define arch_nr (offsetof(struct seccomp_data, arch))
+
+#if defined(__i386__)
+# define ARCH_NR AUDIT_ARCH_I386
+#elif defined(__x86_64__)
+# define ARCH_NR AUDIT_ARCH_X86_64
+#elif defined(__arm__)
+# define ARCH_NR AUDIT_ARCH_ARM
+#else
+# error "Seccomp-bpf sandbox unsupported on this architecture"
+#endif
+
+#define VALIDATE_ARCHITECTURE \
+    BPF_STMT(BPF_LD+BPF_W+BPF_ABS, arch_nr), \
+    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ARCH_NR, 1, 0), \
+    BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL)
+
+#define EXAMINE_SYSCALL \
+    BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr)
+
+#define ALLOW_SYSCALL(name) \
+    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_##name, 0, 1), \
+    BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
+
+#define KILL_PROCESS \
+    BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL)
+
+#define DENY_SYSCALL(name) \
+    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_##name, 0, 1), \
+    BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL_PROCESS)
+
+static sock_filter filter[] = {
+    VALIDATE_ARCHITECTURE,
+    EXAMINE_SYSCALL,
+
+    // examples of undesirable syscalls
+    DENY_SYSCALL(execve),
+    DENY_SYSCALL(fork),
+    DENY_SYSCALL(vfork),
+    DENY_SYSCALL(clone),
+    DENY_SYSCALL(connect),
+    DENY_SYSCALL(listen),
+    DENY_SYSCALL(bind),
+    DENY_SYSCALL(kill),
+    DENY_SYSCALL(settimeofday),
+    // etc
+
+    // default-permit mode
+    BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
+};
+
+static sock_fprog prog = {
+    ARRLEN(filter), filter
+};
+
+void sandbox_start() {
+    if (!settings::SANDBOX) {
+        std::cout << "[WARN] Running without a sandbox" << std::endl;
+        return;
+    }
+
+    std::cout << "[INFO] Starting seccomp-bpf sandbox..." << std::endl;
+
+    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) {
+        perror("prctl");
+        exit(1);
+    }
+
+    if (seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_TSYNC, &prog) < 0) {
+        perror("seccomp");
+        exit(1);
+    }
+}
+
+#endif // SANDBOX_SECCOMP