homework-cifs: Pass password via fd

Pass the password into mount.cifs via a file descriptor, rather
than putting it into a plain-text file in /tmp. This uses the $PASSWD_FD
environment variable, which is undocumented but has existed since
forever (initial commit from 2010 [1] has it already)

[1]:
https://git.samba.org/?p=cifs-utils.git;a=blob;f=mount.cifs.c;hb=ce0b1609a9eedce6c5eb20eab287ea44217c0a6a#l1477

1 files changed, 32 insertions, 30 deletions

diff --git a/src/home/homework-cifs.c b/src/home/homework-cifs.c
index 19f1cd5b85..5d8713111e 100644
--- a/src/home/homework-cifs.c
+++ b/src/home/homework-cifs.c
@@ -5,6 +5,7 @@
 #include <linux/fs.h>
 #endif
 
+#include "data-fd-util.h"
 #include "dirent-util.h"
 #include "fd-util.h"
 #include "fileio.h"
@@ -24,7 +25,7 @@ int home_setup_cifs(
                 HomeSetupFlags flags,
                 HomeSetup *setup) {
 
-        _cleanup_free_ char *chost = NULL, *cservice = NULL, *cdir = NULL, *chost_and_service = NULL, *j = NULL;
+        _cleanup_free_ char *chost = NULL, *cservice = NULL, *cdir = NULL, *chost_and_service = NULL, *j = NULL, *options = NULL;
         int r;
 
         assert(h);
@@ -53,49 +54,50 @@ int home_setup_cifs(
         if (!chost_and_service)
                 return log_oom();
 
+        if (asprintf(&options, "user=%s,uid=" UID_FMT ",forceuid,gid=" GID_FMT ",forcegid,file_mode=0%3o,dir_mode=0%3o",
+                     user_record_cifs_user_name(h), h->uid, user_record_gid(h), user_record_access_mode(h),
+                     user_record_access_mode(h)) < 0)
+                return log_oom();
+
+        if (h->cifs_domain)
+                if (strextendf_with_separator(&options, ",", "domain=%s", h->cifs_domain) < 0)
+                        return log_oom();
+
+        if (h->cifs_extra_mount_options)
+                if (!strextend_with_separator(&options, ",", h->cifs_extra_mount_options))
+                        return log_oom();
+
         r = home_unshare_and_mkdir();
         if (r < 0)
                 return r;
 
         STRV_FOREACH(pw, h->password) {
-                _cleanup_(unlink_and_freep) char *p = NULL;
-                _cleanup_free_ char *options = NULL;
-                _cleanup_fclose_ FILE *f = NULL;
+                _cleanup_close_ int passwd_fd = -EBADF;
                 pid_t mount_pid;
                 int exit_status;
 
-                r = fopen_temporary_child(NULL, &f, &p);
-                if (r < 0)
-                        return log_error_errno(r, "Failed to create temporary credentials file: %m");
-
-                fprintf(f,
-                        "username=%s\n"
-                        "password=%s\n",
-                        user_record_cifs_user_name(h),
-                        *pw);
-
-                if (h->cifs_domain)
-                        fprintf(f, "domain=%s\n", h->cifs_domain);
-
-                r = fflush_and_check(f);
-                if (r < 0)
-                        return log_error_errno(r, "Failed to write temporary credentials file: %m");
-
-                f = safe_fclose(f);
-
-                if (asprintf(&options, "credentials=%s,uid=" UID_FMT ",forceuid,gid=" GID_FMT ",forcegid,file_mode=0%3o,dir_mode=0%3o",
-                             p, h->uid, user_record_gid(h), user_record_access_mode(h), user_record_access_mode(h)) < 0)
-                        return log_oom();
-
-                if (h->cifs_extra_mount_options)
-                        if (!strextend_with_separator(&options, ",", h->cifs_extra_mount_options))
-                                return log_oom();
+                passwd_fd = acquire_data_fd(*pw, strlen(*pw), /* flags= */ 0);
+                if (passwd_fd < 0)
+                        return log_error_errno(passwd_fd, "Failed to create data FD for password: %m");
 
                 r = safe_fork("(mount)", FORK_RESET_SIGNALS|FORK_RLIMIT_NOFILE_SAFE|FORK_DEATHSIG_SIGTERM|FORK_LOG|FORK_STDOUT_TO_STDERR, &mount_pid);
                 if (r < 0)
                         return r;
                 if (r == 0) {
                         /* Child */
+
+                        r = fd_cloexec(passwd_fd, false);
+                        if (r < 0) {
+                                log_error_errno(r, "Failed to disable CLOEXEC on password FD: %m");
+                                _exit(EXIT_FAILURE);
+                        }
+
+                        r = setenvf("PASSWD_FD", /* overwrite= */ true, "%d", passwd_fd);
+                        if (r < 0) {
+                                log_error_errno(errno, "Failed to set $PASSWD_FD: %m");
+                                _exit(EXIT_FAILURE);
+                        }
+
                         execl("/bin/mount", "/bin/mount", "-n", "-t", "cifs",
                               chost_and_service, HOME_RUNTIME_WORK_DIR,
                               "-o", options, NULL);