ssh-generator: add simple new generator

2 files changed, 484 insertions, 0 deletions

diff --git a/src/ssh-generator/meson.build b/src/ssh-generator/meson.build
new file mode 100644
index 0000000000..d21a7a36d9
--- /dev/null
+++ b/src/ssh-generator/meson.build
@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+executables += [
+        generator_template + {
+                'name' : 'systemd-ssh-generator',
+                'sources' : files('ssh-generator.c'),
+        },
+]
diff --git a/src/ssh-generator/ssh-generator.c b/src/ssh-generator/ssh-generator.c
new file mode 100644
index 0000000000..feb967be74
--- /dev/null
+++ b/src/ssh-generator/ssh-generator.c
@@ -0,0 +1,476 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <fcntl.h>
+#include <sys/ioctl.h>
+#include <unistd.h>
+
+#include "creds-util.h"
+#include "fd-util.h"
+#include "fileio.h"
+#include "generator.h"
+#include "install.h"
+#include "missing_socket.h"
+#include "parse-util.h"
+#include "path-util.h"
+#include "proc-cmdline.h"
+#include "socket-netlink.h"
+#include "socket-util.h"
+#include "special.h"
+#include "virt.h"
+
+/* A small generator binding potentially five or more SSH sockets:
+ *
+ *     1. Listen on AF_VSOCK port 22 if we run in a VM with AF_VSOCK enabled
+ *     2. Listen on AF_UNIX socket /run/host/unix-export/ssh if we run in a container with /run/host/ support
+ *     3. Listen on AF_UNIX socket /run/ssh-unix-local/socket (always)
+ *     4. Listen on any socket specified via kernel command line option systemd.ssh_listen=
+ *     5. Similar, but from system credential ssh.listen
+ *
+ * The first two provide a nice way for hosts to connect to containers and VMs they invoke via the usual SSH
+ * logic, but without waiting for networking or suchlike. The third allows the same for local clients. */
+
+static const char *arg_dest = NULL;
+static bool arg_auto = true;
+static char **arg_listen_extra = NULL;
+
+static int parse_proc_cmdline_item(const char *key, const char *value, void *data) {
+        int r;
+
+        assert(key);
+
+        if (proc_cmdline_key_streq(key, "systemd.ssh_auto")) {
+                r = value ? parse_boolean(value) : 1;
+                if (r < 0)
+                        log_warning_errno(r, "Failed to parse systemd.ssh_auto switch \"%s\", ignoring: %m", value);
+                else
+                        arg_auto = r;
+
+        } else if (proc_cmdline_key_streq(key, "systemd.ssh_listen")) {
+
+                if (proc_cmdline_value_missing(key, value))
+                        return 0;
+
+                SocketAddress sa;
+                r = socket_address_parse(&sa, value);
+                if (r < 0)
+                        log_warning_errno(r, "Failed to parse systemd.ssh_listen= expression, ignoring: %s", value);
+                else {
+                        _cleanup_free_ char *s = NULL;
+                        r = socket_address_print(&sa, &s);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to format socket address: %m");
+
+                        if (strv_consume(&arg_listen_extra, TAKE_PTR(s)) < 0)
+                                return log_oom();
+                }
+        }
+
+        return 0;
+}
+
+static int make_sshd_template_unit(
+                const char *dest,
+                const char *template,
+                const char *sshd_binary,
+                const char *found_sshd_template_service,
+                char **generated_sshd_template_unit) {
+
+        int r;
+
+        assert(dest);
+        assert(template);
+        assert(sshd_binary);
+        assert(generated_sshd_template_unit);
+
+        /* If the system has a suitable template already, symlink it to the name we want to reuse it */
+        if (found_sshd_template_service)
+                return generator_add_symlink(
+                                dest,
+                                template,
+                                /* dep_type= */ NULL,
+                                found_sshd_template_service);
+
+        if (!*generated_sshd_template_unit) {
+                _cleanup_fclose_ FILE *f = NULL;
+
+                r = generator_open_unit_file_full(
+                                dest,
+                                /* source= */ NULL,
+                                "sshd-generated@.service", /* Give this generated unit a generic name, since we want to use it for both AF_UNIX and AF_VSOCK */
+                                &f,
+                                generated_sshd_template_unit,
+                                /* ret_temp_path= */ NULL);
+                if (r < 0)
+                        return r;
+
+                fprintf(f,
+                        "[Unit]\n"
+                        "Description=OpenSSH Per-Connection Server Daemon\n"
+                        "Documentation=man:systemd-ssh-generator(8) man:sshd(8)\n"
+                        "[Service]\n"
+                        "ExecStart=-%s -i\n"
+                        "StandardInput=socket",
+                        sshd_binary);
+
+                r = fflush_and_check(f);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to write sshd template: %m");
+        }
+
+        return generator_add_symlink(
+                        dest,
+                        template,
+                        /* dep_type= */ NULL,
+                        *generated_sshd_template_unit);
+}
+
+static int write_socket_unit(
+                const char *dest,
+                const char *unit,
+                const char *listen_stream,
+                const char *comment) {
+
+        int r;
+
+        assert(dest);
+        assert(unit);
+        assert(listen_stream);
+        assert(comment);
+
+        _cleanup_fclose_ FILE *f = NULL;
+        r = generator_open_unit_file(
+                        dest,
+                        /* source= */ NULL,
+                        unit,
+                        &f);
+        if (r < 0)
+                return r;
+
+        fprintf(f,
+                "[Unit]\n"
+                "Description=OpenSSH Server Socket (systemd-ssh-generator, %s)\n"
+                "Documentation=man:systemd-ssh-generator(8)\n"
+                "\n[Socket]\n"
+                "ListenStream=%s\n"
+                "Accept=yes\n"
+                "PollLimitIntervalSec=30s\n"
+                "PollLimitBurst=50\n",
+                comment,
+                listen_stream);
+
+        r = fflush_and_check(f);
+        if (r < 0)
+                return log_error_errno(r, "Failed to write %s SSH socket unit: %m", comment);
+
+        r = generator_add_symlink(
+                        dest,
+                        SPECIAL_SOCKETS_TARGET,
+                        "wants",
+                        unit);
+        if (r < 0)
+                return r;
+
+        return 0;
+}
+
+static int add_vsock_socket(
+                const char *dest,
+                const char *sshd_binary,
+                const char *found_sshd_template_unit,
+                char **generated_sshd_template_unit) {
+
+        int r;
+
+        assert(dest);
+        assert(generated_sshd_template_unit);
+
+        Virtualization v = detect_vm();
+        if (v < 0)
+                return log_error_errno(v, "Failed to detect if we run in a VM: %m");
+        if (v == VIRTUALIZATION_NONE) {
+                log_debug("Not running in a VM, not listening on AF_VSOCK.");
+                return 0;
+        }
+
+        _cleanup_close_ int vsock_fd = socket(AF_VSOCK, SOCK_STREAM|SOCK_CLOEXEC, 0);
+        if (vsock_fd < 0) {
+                if (ERRNO_IS_NOT_SUPPORTED(errno)) {
+                        log_debug("Not creating AF_VSOCK ssh listener, since AF_VSOCK is not available.");
+                        return 0;
+                }
+
+                return log_error_errno(errno, "Unable to test if AF_VSOCK is available: %m");
+        }
+
+        vsock_fd = safe_close(vsock_fd);
+
+        /* Determine the local CID so that we can log it to help users to connect to this VM */
+        unsigned local_cid;
+        r = vsock_get_local_cid(&local_cid);
+        if (r < 0)
+                return log_error_errno(r, "Failed to query local AF_VSOCK CID: %m");
+
+        r = make_sshd_template_unit(
+                        dest,
+                        "sshd-vsock@.service",
+                        sshd_binary,
+                        found_sshd_template_unit,
+                        generated_sshd_template_unit);
+        if (r < 0)
+                return r;
+
+        r = write_socket_unit(
+                        dest,
+                        "sshd-vsock.socket",
+                        "vsock::22",
+                        "AF_VSOCK");
+        if (r < 0)
+                return r;
+
+        log_info("Binding SSH to AF_VSOCK vsock::22.\n"
+                 "→ connect via 'ssh vsock/%u' from host", local_cid);
+        return 0;
+}
+
+static int add_local_unix_socket(
+                const char *dest,
+                const char *sshd_binary,
+                const char *found_sshd_template_unit,
+                char **generated_sshd_template_unit) {
+
+        int r;
+
+        assert(dest);
+        assert(sshd_binary);
+        assert(generated_sshd_template_unit);
+
+        r = make_sshd_template_unit(
+                        dest,
+                        "sshd-unix-local@.service",
+                        sshd_binary,
+                        found_sshd_template_unit,
+                        generated_sshd_template_unit);
+        if (r < 0)
+                return r;
+
+        r = write_socket_unit(
+                        dest,
+                        "sshd-unix-local.socket",
+                        "/run/ssh-unix-local/socket",
+                        "AF_UNIX Local");
+        if (r < 0)
+                return r;
+
+
+        log_info("Binding SSH to AF_UNIX socket /run/ssh-unix-local/socket.\n"
+                 "→ connect via 'ssh .host' locally");
+        return 0;
+}
+
+static int add_export_unix_socket(
+                const char *dest,
+                const char *sshd_binary,
+                const char *found_sshd_template_unit,
+                char **generated_sshd_template_unit) {
+
+        int r;
+
+        assert(dest);
+        assert(sshd_binary);
+        assert(generated_sshd_template_unit);
+
+        Virtualization v = detect_container();
+        if (v < 0)
+                return log_error_errno(v, "Failed to detect if we run in a container: %m");
+        if (v == VIRTUALIZATION_NONE) {
+                log_debug("Not running in container, not listening on /run/host/unix-export/ssh");
+                return 0;
+        }
+
+        if (access("/run/host/unix-export/", W_OK) < 0) {
+                if (errno == ENOENT) {
+                        log_debug("Container manager does not provide /run/host/unix-export/ mount, not binding AF_UNIX socket there.");
+                        return 0;
+                }
+                if (errno == EROFS || ERRNO_IS_PRIVILEGE(errno)) {
+                        log_debug("Container manager does not provide write access to /run/host/unix-export/, not binding AF_UNIX socket there.");
+                        return 0;
+                }
+
+                return log_debug_errno(errno, "Unable to check if /run/host/unix-export exists: %m");
+        }
+
+        r = make_sshd_template_unit(
+                        dest,
+                        "sshd-unix-export@.service",
+                        sshd_binary,
+                        found_sshd_template_unit,
+                        generated_sshd_template_unit);
+        if (r < 0)
+                return r;
+
+        r = write_socket_unit(
+                        dest,
+                        "sshd-unix-export.socket",
+                        "/run/host/unix-export/ssh",
+                        "AF_UNIX Export");
+        if (r < 0)
+                return r;
+
+        log_info("Binding SSH to AF_UNIX socket /run/host/unix-export/ssh\n"
+                 "→ connect via 'ssh unix/run/systemd/nspawn/unix-export/\?\?\?/ssh' from host");
+
+        return 0;
+}
+
+static int add_extra_sockets(
+                const char *dest,
+                const char *sshd_binary,
+                const char *found_sshd_template_unit,
+                char **generated_sshd_template_unit) {
+
+        unsigned n = 1;
+        int r;
+
+        assert(dest);
+        assert(sshd_binary);
+        assert(generated_sshd_template_unit);
+
+        if (strv_isempty(arg_listen_extra))
+                return 0;
+
+        STRV_FOREACH(i, arg_listen_extra) {
+                _cleanup_free_ char *service = NULL, *socket = NULL;
+
+                if (n > 1) {
+                        if (asprintf(&service, "sshd-extra-%u@.service", n) < 0)
+                                return log_oom();
+
+                        if (asprintf(&socket, "sshd-extra-%u.socket", n) < 0)
+                                return log_oom();
+                }
+
+                r = make_sshd_template_unit(
+                                dest,
+                                service ?: "sshd-extra@.service",
+                                sshd_binary,
+                                found_sshd_template_unit,
+                                generated_sshd_template_unit);
+                if (r < 0)
+                        return r;
+
+                r = write_socket_unit(
+                                dest,
+                                socket ?: "sshd-extra.socket",
+                                *i,
+                                *i);
+                if (r < 0)
+                        return r;
+
+                log_info("Binding SSH to socket %s.", *i);
+                n++;
+        }
+
+        return 0;
+}
+
+static int parse_credentials(void) {
+        _cleanup_free_ char *b = NULL;
+        size_t sz = 0;
+        int r;
+
+        r = read_credential_with_decryption("ssh.listen", (void*) &b, &sz);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return 0;
+
+        _cleanup_fclose_ FILE *f = NULL;
+        f = fmemopen_unlocked(b, sz, "r");
+        if (!f)
+                return log_oom();
+
+        for (;;) {
+                _cleanup_free_ char *item = NULL;
+
+                r = read_stripped_line(f, LINE_MAX, &item);
+                if (r == 0)
+                        break;
+                if (r < 0) {
+                        log_error_errno(r, "Failed to parse credential 'ssh.listen': %m");
+                        break;
+                }
+
+                if (startswith(item, "#"))
+                        continue;
+
+                SocketAddress sa;
+                r = socket_address_parse(&sa, item);
+                if (r < 0) {
+                        log_warning_errno(r, "Failed to parse systemd.ssh_listen= expression, ignoring: %s", item);
+                        continue;
+                }
+
+                _cleanup_free_ char *s = NULL;
+                r = socket_address_print(&sa, &s);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to format socket address: %m");
+
+                if (strv_consume(&arg_listen_extra, TAKE_PTR(s)) < 0)
+                        return log_oom();
+        }
+
+        return 0;
+}
+
+static int run(const char *dest, const char *dest_early, const char *dest_late) {
+        int r;
+
+        assert_se(arg_dest = dest);
+
+        r = proc_cmdline_parse(parse_proc_cmdline_item, /* userdata= */ NULL, /* flags= */ 0);
+        if (r < 0)
+                log_warning_errno(r, "Failed to parse kernel command line, ignoring: %m");
+
+        (void) parse_credentials();
+
+        strv_sort(arg_listen_extra);
+        strv_uniq(arg_listen_extra);
+
+        if (!arg_auto && strv_isempty(arg_listen_extra)) {
+                log_debug("Disabling SSH generator logic, because as it has been turned off explicitly.");
+                return 0;
+        }
+
+        _cleanup_free_ char *sshd_binary = NULL;
+        r = find_executable("sshd", &sshd_binary);
+        if (r == -ENOENT) {
+                log_info("Disabling SSH generator logic, since sshd is not installed.");
+                return 0;
+        }
+        if (r < 0)
+                return log_error_errno(r, "Failed to determine if sshd is installed: %m");
+
+        _cleanup_(lookup_paths_free) LookupPaths lp = {};
+        r = lookup_paths_init_or_warn(&lp, RUNTIME_SCOPE_SYSTEM, LOOKUP_PATHS_EXCLUDE_GENERATED, /* root_dir= */ NULL);
+        if (r < 0)
+                return r;
+
+        _cleanup_free_ char *found_sshd_template_unit = NULL;
+        r = unit_file_exists_full(RUNTIME_SCOPE_SYSTEM, &lp, "sshd@.service", &found_sshd_template_unit);
+        if (r < 0)
+                return log_error_errno(r, "Unable to detect if sshd@.service exists: %m");
+
+        _cleanup_free_ char *generated_sshd_template_unit = NULL;
+        RET_GATHER(r, add_extra_sockets(dest, sshd_binary, found_sshd_template_unit, &generated_sshd_template_unit));
+
+        if (arg_auto) {
+                RET_GATHER(r, add_vsock_socket(dest, sshd_binary, found_sshd_template_unit, &generated_sshd_template_unit));
+                RET_GATHER(r, add_local_unix_socket(dest, sshd_binary, found_sshd_template_unit, &generated_sshd_template_unit));
+                RET_GATHER(r, add_export_unix_socket(dest, sshd_binary, found_sshd_template_unit, &generated_sshd_template_unit));
+        }
+
+        return r;
+}
+
+DEFINE_MAIN_GENERATOR_FUNCTION(run);