dev-setup: generalize logic we use to create "inaccessible" device nodes

Let's generalize this, so that we can use this in nspawn later on, which
is pretty useful as we need to be able to mask files from the inner
child of nspawn too, where the host's /run/systemd/inaccessible
directory is not visible anymore. Moreover, if nspawn can create these
nodes on its own before the payload this means the payload can run with
fewer privileges.

src/core/mount-setup.c

@@ -460,20 +460,9 @@ int mount_setup(bool loaded_policy) {
         (void) mkdir_label("/run/systemd", 0755);
         (void) mkdir_label("/run/systemd/system", 0755);
 
-        /* Set up inaccessible (and empty) file nodes of all types */
+        /* Also create /run/systemd/inaccessible nodes, so that we always have something to mount inaccessible nodes
-        (void) mkdir_label("/run/systemd/inaccessible", 0000);
+         * from. */
-        (void) mknod("/run/systemd/inaccessible/reg", S_IFREG | 0000, 0);
+        (void) make_inaccessible_nodes(NULL, UID_INVALID, GID_INVALID);
-        (void) mkdir_label("/run/systemd/inaccessible/dir", 0000);
-        (void) mkfifo("/run/systemd/inaccessible/fifo", 0000);
-        (void) mknod("/run/systemd/inaccessible/sock", S_IFSOCK | 0000, 0);
-
-        /* The following two are likely to fail if we lack the privs for it (for example in an userns environment, if
-         * CAP_SYS_MKNOD is missing, or if a device node policy prohibit major/minor of 0 device nodes to be
-         * created). But that's entirely fine. Consumers of these files should carry fallback to use a different node
-         * then, for example /run/systemd/inaccessible/sock, which is close enough in behaviour and semantics for most
-         * uses. */
-        (void) mknod("/run/systemd/inaccessible/chr", S_IFCHR | 0000, makedev(0, 0));
-        (void) mknod("/run/systemd/inaccessible/blk", S_IFBLK | 0000, makedev(0, 0));
 
         return 0;
 }

src/shared/dev-setup.c

@@ -9,6 +9,7 @@
 #include "label.h"
 #include "log.h"
 #include "path-util.h"
+#include "umask-util.h"
 #include "user-util.h"
 #include "util.h"
 
@@ -54,3 +55,61 @@ int dev_setup(const char *prefix, uid_t uid, gid_t gid) {
 
         return 0;
 }
+
+int make_inaccessible_nodes(const char *root, uid_t uid, gid_t gid) {
+        static const struct {
+                const char *name;
+                mode_t mode;
+        } table[] = {
+                { "/run/systemd",                   S_IFDIR  | 0755 },
+                { "/run/systemd/inaccessible",      S_IFDIR  | 0000 },
+                { "/run/systemd/inaccessible/reg",  S_IFREG  | 0000 },
+                { "/run/systemd/inaccessible/dir",  S_IFDIR  | 0000 },
+                { "/run/systemd/inaccessible/fifo", S_IFIFO  | 0000 },
+                { "/run/systemd/inaccessible/sock", S_IFSOCK | 0000 },
+
+                /* The following two are likely to fail if we lack the privs for it (for example in an userns
+                 * environment, if CAP_SYS_MKNOD is missing, or if a device node policy prohibit major/minor of 0
+                 * device nodes to be created). But that's entirely fine. Consumers of these files should carry
+                 * fallback to use a different node then, for example /run/systemd/inaccessible/sock, which is close
+                 * enough in behaviour and semantics for most uses. */
+                { "/run/systemd/inaccessible/chr",  S_IFCHR  | 0000 },
+                { "/run/systemd/inaccessible/blk",  S_IFBLK  | 0000 },
+        };
+
+        _cleanup_umask_ mode_t u;
+        size_t i;
+        int r;
+
+        u = umask(0000);
+
+        /* Set up inaccessible (and empty) file nodes of all types. This are used to as mount sources for over-mounting
+         * ("masking") file nodes that shall become inaccessible and empty for specific containers or services. We try
+         * to lock down these nodes as much as we can, but otherwise try to match them as closely as possible with the
+         * underlying file, i.e. in the best case we offer the same node type as the underlying node. */
+
+        for (i = 0; i < ELEMENTSOF(table); i++) {
+                _cleanup_free_ char *path = NULL;
+
+                path = prefix_root(root, table[i].name);
+                if (!path)
+                        return log_oom();
+
+                if (S_ISDIR(table[i].mode))
+                        r = mkdir(path, table[i].mode & 07777);
+                else
+                        r = mknod(path, table[i].mode, makedev(0, 0));
+                if (r < 0) {
+                        if (errno != EEXIST)
+                                log_debug_errno(errno, "Failed to create '%s', ignoring: %m", path);
+                        continue;
+                }
+
+                if (uid != UID_INVALID || gid != GID_INVALID) {
+                        if (lchown(path, uid, gid) < 0)
+                                log_debug_errno(errno, "Failed to chown '%s': %m", path);
+                }
+        }
+
+        return 0;
+}

src/shared/dev-setup.h

@@ -4,3 +4,5 @@
 #include <sys/types.h>
 
 int dev_setup(const char *prefix, uid_t uid, gid_t gid);
+
+int make_inaccessible_nodes(const char *root, uid_t uid, gid_t gid);

src/test/meson.build

@@ -156,6 +156,10 @@ tests += [
          [],
          []],
 
+        [['src/test/test-dev-setup.c'],
+         [],
+         []],
+
         [['src/test/test-capability.c'],
          [],
          [libcap]],

src/test/test-dev-setup.c

@@ -0,0 +1,62 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#include "capability-util.h"
+#include "dev-setup.h"
+#include "fileio.h"
+#include "fs-util.h"
+#include "path-util.h"
+#include "rm-rf.h"
+
+int main(int argc, char *argv[]) {
+        _cleanup_(rm_rf_physical_and_freep) char *p = NULL;
+        const char *f;
+        struct stat st;
+
+        if (have_effective_cap(CAP_DAC_OVERRIDE) <= 0)
+                return EXIT_TEST_SKIP;
+
+        assert_se(mkdtemp_malloc("/tmp/test-dev-setupXXXXXX", &p) >= 0);
+
+        f = prefix_roota(p, "/run");
+        assert_se(mkdir(f, 0755) >= 0);
+
+        assert_se(make_inaccessible_nodes(p, 1, 1) >= 0);
+
+        f = prefix_roota(p, "/run/systemd/inaccessible/reg");
+        assert_se(stat(f, &st) >= 0);
+        assert_se(S_ISREG(st.st_mode));
+        assert_se((st.st_mode & 07777) == 0000);
+
+        f = prefix_roota(p, "/run/systemd/inaccessible/dir");
+        assert_se(stat(f, &st) >= 0);
+        assert_se(S_ISDIR(st.st_mode));
+        assert_se((st.st_mode & 07777) == 0000);
+
+        f = prefix_roota(p, "/run/systemd/inaccessible/fifo");
+        assert_se(stat(f, &st) >= 0);
+        assert_se(S_ISFIFO(st.st_mode));
+        assert_se((st.st_mode & 07777) == 0000);
+
+        f = prefix_roota(p, "/run/systemd/inaccessible/sock");
+        assert_se(stat(f, &st) >= 0);
+        assert_se(S_ISSOCK(st.st_mode));
+        assert_se((st.st_mode & 07777) == 0000);
+
+        f = prefix_roota(p, "/run/systemd/inaccessible/chr");
+        if (stat(f, &st) < 0)
+                assert_se(errno == ENOENT);
+        else {
+                assert_se(S_ISCHR(st.st_mode));
+                assert_se((st.st_mode & 07777) == 0000);
+        }
+
+        f = prefix_roota(p, "/run/systemd/inaccessible/blk");
+        if (stat(f, &st) < 0)
+                assert_se(errno == ENOENT);
+        else {
+                assert_se(S_ISBLK(st.st_mode));
+                assert_se((st.st_mode & 07777) == 0000);
+        }
+
+        return EXIT_SUCCESS;
+}