nspawn: be less liberal about creating bind mount destinations

Previously, if a file's bind mount destination didn't exist, nspawn
would blindly create a directory, and the subsequent bind mount would
fail. Examine the filetype of the source and ensure that, if the
destination does not exist, that it is created appropriately.

Also go one step further and ensure that the filetypes of the source
and destination match.

1 files changed, 28 insertions, 1 deletions

diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index f8208607ce..a0809da743 100644
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -411,12 +411,39 @@ static int mount_binds(const char *dest, char **l, unsigned long flags) {
 
         STRV_FOREACH_PAIR(x, y, l) {
                 _cleanup_free_ char *where = NULL;
+                struct stat source_st, dest_st;
+
+                if (stat(*x, &source_st) < 0) {
+                        log_error("failed to stat %s: %m", *x);
+                        return -errno;
+                }
 
                 where = strjoin(dest, "/", *y, NULL);
                 if (!where)
                         return log_oom();
 
-                mkdir_p_label(where, 0755);
+                if (stat(where, &dest_st) == 0) {
+                        if ((source_st.st_mode & S_IFMT) != (dest_st.st_mode & S_IFMT)) {
+                                log_error("The file types of %s and %s do not matching. Refusing bind mount",
+                                                *x, where);
+                                return -EINVAL;
+                        }
+                } else {
+                        /* Create the mount point, but be conservative -- refuse to create block
+                         * and char devices. */
+                        if (S_ISDIR(source_st.st_mode))
+                                mkdir_p_label(where, 0755);
+                        else if (S_ISFIFO(source_st.st_mode))
+                                mkfifo(where, 0644);
+                        else if (S_ISSOCK(source_st.st_mode))
+                                mknod(where, 0644 | S_IFSOCK, 0);
+                        else if (S_ISREG(source_st.st_mode))
+                                touch(where);
+                        else {
+                                log_error("Refusing to create mountpoint for file: %s", *x);
+                                return -ENOTSUP;
+                        }
+                }
 
                 if (mount(*x, where, "bind", MS_BIND, NULL) < 0) {
                         log_error("mount(%s) failed: %m", where);