nspawn: Prevent invalid UIDs propagating in bind mounts (#39729)

Commit 88fce09026 modified the
mount_bind() function, causing it to perform arithmetic on the uid_shift
parameter. However, it performs this arithmetic even when uid_shift was
UID_INVALID, which was not intended. This typically occurred when
mount_custom() was called for a simple bind mount without user
namespaces (and thus no rootidmap mount option).

This arithmetic (e.g., uid_shift + m->destination_uid) then wraps
around, resulting in the invalid ID 4294967295 ((uid_t)-1).

This bug manifests for users running systemd-nspawn with
--link-journal=host and --volatile=yes (but without --private-users),
causing systemd-tmpfiles to fail.

Make mount_bind() robust by checking if uid_shift is valid before using
it in arithmetic. If it is UID_INVALID, it defaults to a shift of 0 for
the ownership calculation, restoring correct behavior for plain bind
mounts while preserving the intended logic for ID-mapped mounts.

Fixes: #39714

src/nspawn/nspawn-mount.c

@@ -814,9 +814,20 @@ static int mount_bind(const char *dest, CustomMount *m, uid_t uid_shift, uid_t u
                         return r;
         }
 
+        /* ID remapping cannot be done if user namespaces are not in use (uid_shift is UID_INVALID).
+         * Fail if idmapping was explicitly requested in this state. Otherwise, treat UID_INVALID
+         * as 0 for ownership operations. */
+        if (idmapping != REMOUNT_IDMAPPING_NONE && !uid_is_valid(uid_shift))
+                return log_error_errno(
+                                SYNTHETIC_ERRNO(EINVAL),
+                                "ID remapping requested for %s, but user namespacing is not enabled.",
+                                m->source);
+
+        uid_t chown_uid = uid_is_valid(uid_shift) ? uid_shift : 0;
+
         /* If this is a bind mount from a temporary sources change ownership of the source to the container's
          * root UID. Otherwise it would always show up as "nobody" if user namespacing is used. */
-        if (m->rm_rf_tmpdir && chown(m->source, uid_shift, uid_shift) < 0)
+        if (m->rm_rf_tmpdir && chown(m->source, chown_uid, chown_uid) < 0)
                 return log_error_errno(errno, "Failed to chown %s: %m", m->source);
 
         /* UID/GIDs of idmapped mounts are always resolved in the caller's user namespace. In other
@@ -850,7 +861,7 @@ static int mount_bind(const char *dest, CustomMount *m, uid_t uid_shift, uid_t u
                 if (stat(where, &dest_st) < 0)
                         return log_error_errno(errno, "Failed to stat %s: %m", where);
 
-                dest_uid = uid_is_valid(m->destination_uid) ? uid_shift + m->destination_uid : dest_st.st_uid;
+                dest_uid = uid_is_valid(m->destination_uid) ? chown_uid + m->destination_uid : dest_st.st_uid;
 
                 if (S_ISDIR(source_st.st_mode) && !S_ISDIR(dest_st.st_mode))
                         return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
@@ -863,7 +874,7 @@ static int mount_bind(const char *dest, CustomMount *m, uid_t uid_shift, uid_t u
                                                m->source, where);
 
         } else { /* Path doesn't exist yet? */
-                r = mkdir_parents_safe_label(dest, where, 0755, uid_shift, uid_shift, MKDIR_IGNORE_EXISTING);
+                r = mkdir_parents_safe_label(dest, where, 0755, chown_uid, chown_uid, MKDIR_IGNORE_EXISTING);
                 if (r < 0)
                         return log_error_errno(r, "Failed to make parents of %s: %m", where);
 
@@ -878,10 +889,10 @@ static int mount_bind(const char *dest, CustomMount *m, uid_t uid_shift, uid_t u
                 if (r < 0)
                         return log_error_errno(r, "Failed to create mount point %s: %m", where);
 
-                if (chown(where, uid_shift, uid_shift) < 0)
+                if (chown(where, chown_uid, chown_uid) < 0)
                         return log_error_errno(errno, "Failed to chown %s: %m", where);
 
-                dest_uid = uid_shift + (uid_is_valid(m->destination_uid) ? m->destination_uid : 0);
+                dest_uid = chown_uid + (uid_is_valid(m->destination_uid) ? m->destination_uid : 0);
         }
 
         if (move_mount(fd_clone, "", AT_FDCWD, where, MOVE_MOUNT_F_EMPTY_PATH) < 0)

test/units/TEST-13-NSPAWN.nspawn.sh

@@ -1477,6 +1477,44 @@ testcase_link_journal_host() {
     rm -fr "$root" "$hoge"
 }
 
+testcase_volatile_link_journal_no_userns() {
+    local root machine_id journal_dir acl_output
+
+    root="$(mktemp -d /var/lib/machines/TEST-13-NSPAWN.volatile-journal.XXX)"
+    create_dummy_container "$root"
+
+    machine_id="$(systemd-id128 new)"
+    echo "$machine_id" >"$root/etc/machine-id"
+
+    journal_dir="/var/log/journal/$machine_id"
+    mkdir -p "$journal_dir"
+    chown root:root "$journal_dir"
+
+    systemd-nspawn --register=no \
+                   --directory="$root" \
+                   --boot \
+                   --volatile=yes \
+                   --link-journal=host \
+                   systemd.unit=systemd-tmpfiles-setup.service
+
+    local gid
+    gid="$(stat -c '%g' "$journal_dir")"
+
+    # Ensure GID is not 4294967295 (GID_INVALID)
+    [[ "$gid" != "4294967295" ]]
+
+    # Ensure the directory is owned by a valid user (root or systemd-journal
+    # group). The GID should be either 0 (root) or the systemd-journal GID, not
+    # some bombastically large number
+    [[ "$gid" -lt 65535 ]]
+
+    # Ensure the invalid GID doesn't appear in ACLs
+    acl_output="$(getfacl "$journal_dir" || true)"
+    grep -q "4294967295" <<< "$acl_output" && exit 1
+
+    rm -fr "$root" "$journal_dir"
+}
+
 testcase_cap_net_bind_service() {
     local root