src/shared/switch-root.c

   1 /* SPDX-License-Identifier: LGPL-2.1-or-later */
   2
   3 #include <errno.h>
   4 #include <fcntl.h>
   5 #include <limits.h>
   6 #include <stdbool.h>
   7 #include <sys/mount.h>
   8 #include <sys/stat.h>
   9 #include <unistd.h>
  10
  11 #include "base-filesystem.h"
  12 #include "chase.h"
  13 #include "creds-util.h"
  14 #include "fd-util.h"
  15 #include "initrd-util.h"
  16 #include "log.h"
  17 #include "missing_syscall.h"
  18 #include "mkdir-label.h"
  19 #include "mount-util.h"
  20 #include "mountpoint-util.h"
  21 #include "path-util.h"
  22 #include "rm-rf.h"
  23 #include "stdio-util.h"
  24 #include "string-util.h"
  25 #include "strv.h"
  26 #include "switch-root.h"
  27 #include "user-util.h"
  28
  29 int switch_root(const char *new_root,
  30                 const char *old_root_after,   /* path below the new root, where to place the old root after the transition; may be NULL to unmount it */
  31                 SwitchRootFlags flags) {
  32
  33         /* Stuff mounted below /run we don't save on soft reboot, as it might have lost its relevance, i.e.
  34          * credentials, removable media and such, we rather want that the new boot mounts this fresh.
  35          * But on the switch from initrd we do use MS_REC, as it is expected that mounts set up in /run
  36          * are maintained. */
  37         unsigned long run_mount_flags = MS_BIND|(!FLAGS_SET(flags, SWITCH_ROOT_SKIP_RECURSIVE_RUN) ? MS_REC : 0);
  38         struct {
  39                 const char *path;
  40                 unsigned long mount_flags;
  41                 bool skip_if_run_is_rec; /* For child mounts of /run, if it's moved recursively no need to handle */
  42         } transfer_table[] = {
  43                 { "/dev",                                 MS_BIND|MS_REC,  false }, /* Recursive, because we want to save the original /dev/shm + /dev/pts and similar */
  44                 { "/sys",                                 MS_BIND|MS_REC,  false }, /* Similar, we want to retain various API VFS, or the cgroupv1 /sys/fs/cgroup/ tree */
  45                 { "/proc",                                MS_BIND|MS_REC,  false }, /* Similar */
  46                 { "/run",                                 run_mount_flags, false }, /* Recursive except on soft reboot, see above */
  47                 { SYSTEM_CREDENTIALS_DIRECTORY,           MS_BIND,         true },  /* Credentials passed into the system should survive */
  48                 { ENCRYPTED_SYSTEM_CREDENTIALS_DIRECTORY, MS_BIND,         true },  /* Similar */
  49                 { "/run/host",                            MS_BIND|MS_REC,  true },  /* Host supplied hierarchy should also survive */
  50         };
  51
  52         _cleanup_close_ int old_root_fd = -EBADF, new_root_fd = -EBADF;
  53         _cleanup_free_ char *resolved_old_root_after = NULL;
  54         int r, istmp;
  55
  56         assert(new_root);
  57
  58         /* Check if we shall remove the contents of the old root */
  59         old_root_fd = open("/", O_DIRECTORY|O_CLOEXEC);
  60         if (old_root_fd < 0)
  61                 return log_error_errno(errno, "Failed to open root directory: %m");
  62
  63         new_root_fd = open(new_root, O_DIRECTORY|O_CLOEXEC);
  64         if (new_root_fd < 0)
  65                 return log_error_errno(errno, "Failed to open target directory '%s': %m", new_root);
  66
  67         r = inode_same_at(old_root_fd, "", new_root_fd, "", AT_EMPTY_PATH);
  68         if (r < 0)
  69                 return log_error_errno(r, "Failed to determine if old and new root directory are the same: %m");
  70         if (r > 0) {
  71                 log_debug("Skipping switch root, as old and new root directory are the same.");
  72                 return 0;
  73         }
  74
  75         /* Make the new root directory a mount point if it isn't */
  76         r = fd_make_mount_point(new_root_fd);
  77         if (r < 0)
  78                 return log_error_errno(r, "Failed to make new root directory a mount point: %m");
  79
  80         if (FLAGS_SET(flags, SWITCH_ROOT_DESTROY_OLD_ROOT)) {
  81                 istmp = fd_is_temporary_fs(old_root_fd);
  82                 if (istmp < 0)
  83                         return log_error_errno(istmp, "Failed to stat root directory: %m");
  84                 if (istmp > 0)
  85                         log_debug("Root directory is on tmpfs, will do cleanup later.");
  86         } else
  87                 istmp = -1; /* don't know */
  88
  89         if (old_root_after) {
  90                 /* Determine where we shall place the old root after the transition */
  91                 r = chase(old_root_after, new_root, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &resolved_old_root_after, NULL);
  92                 if (r < 0)
  93                         return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, old_root_after);
  94                 if (r == 0) /* Doesn't exist yet. Let's create it */
  95                         (void) mkdir_p_label(resolved_old_root_after, 0755);
  96         }
  97
  98         /* We are about to unmount various file systems with MNT_DETACH (either explicitly via umount() or
  99          * indirectly via pivot_root()), and thus do not synchronously wait for them to be fully sync'ed —
 100          * all while making them invisible/inaccessible in the file system tree for later code. That makes
 101          * sync'ing them then difficult. Let's hence issue a manual sync() here, so that we at least can
 102          * guarantee all file systems are an a good state before entering this state. */
 103         if (!FLAGS_SET(flags, SWITCH_ROOT_DONT_SYNC))
 104                 sync();
 105
 106         /* Work-around for kernel design: the kernel refuses MS_MOVE if any file systems are mounted
 107          * MS_SHARED. Hence remount them MS_PRIVATE here as a work-around.
 108          *
 109          * https://bugzilla.redhat.com/show_bug.cgi?id=847418 */
 110         if (mount(NULL, "/", NULL, MS_REC|MS_PRIVATE, NULL) < 0)
 111                 return log_error_errno(errno, "Failed to set \"/\" mount propagation to private: %m");
 112
 113         /* Do not fail if base_filesystem_create() fails. Not all switch roots are like base_filesystem_create() wants
 114          * them to look like. They might even boot, if they are RO and don't have the FS layout. Just ignore the error
 115          * and switch_root() nevertheless. */
 116         (void) base_filesystem_create_fd(new_root_fd, new_root, UID_INVALID, GID_INVALID);
 117
 118         FOREACH_ARRAY(transfer, transfer_table, ELEMENTSOF(transfer_table)) {
 119                 _cleanup_free_ char *chased = NULL;
 120
 121                 if (transfer->skip_if_run_is_rec && !FLAGS_SET(flags, SWITCH_ROOT_SKIP_RECURSIVE_RUN))
 122                         continue;
 123
 124                 if (access(transfer->path, F_OK) < 0) {
 125                         log_debug_errno(errno, "Path '%s' to move to target root directory, not found, ignoring: %m", transfer->path);
 126                         continue;
 127                 }
 128
 129                 r = chase(transfer->path, new_root, CHASE_PREFIX_ROOT, &chased, NULL);
 130                 if (r < 0)
 131                         return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, transfer->path);
 132
 133                 /* Let's see if it is a mount point already. */
 134                 r = path_is_mount_point(chased, NULL, 0);
 135                 if (r < 0)
 136                         return log_error_errno(r, "Failed to determine whether %s is a mount point: %m", chased);
 137                 if (r > 0) /* If it is already mounted, then do nothing */
 138                         continue;
 139
 140                 r = mount_nofollow_verbose(LOG_ERR, transfer->path, chased, NULL, transfer->mount_flags, NULL);
 141                 if (r < 0)
 142                         return r;
 143         }
 144
 145         if (fchdir(new_root_fd) < 0)
 146                 return log_error_errno(errno, "Failed to change directory to %s: %m", new_root);
 147
 148         /* We first try a pivot_root() so that we can umount the old root dir. In many cases (i.e. where rootfs is /),
 149          * that's not possible however, and hence we simply overmount root */
 150         if (resolved_old_root_after)
 151                 r = RET_NERRNO(pivot_root(".", resolved_old_root_after));
 152         else {
 153                 r = RET_NERRNO(pivot_root(".", "."));
 154                 if (r >= 0) {
 155                         /* Now unmount the upper of the two stacked file systems */
 156                         if (umount2(".", MNT_DETACH) < 0)
 157                                 return log_error_errno(errno, "Failed to unmount the old root: %m");
 158                 }
 159         }
 160         if (r < 0) {
 161                 log_debug_errno(r, "Pivoting root file system failed, moving mounts instead: %m");
 162
 163                 /* If we have to use MS_MOVE let's first try to get rid of *all* mounts we can, with the
 164                  * exception of the path we want to switch to, plus everything leading to it and within
 165                  * it. This is necessary because unlike pivot_root() just moving the mount to the root via
 166                  * MS_MOVE won't magically unmount anything below it. Once the chroot() succeeds the mounts
 167                  * below would still be around but invisible to us, because not accessible via
 168                  * /proc/self/mountinfo. Hence, let's clean everything up first, as long as we still can. */
 169                 (void) umount_recursive_full(NULL, MNT_DETACH, STRV_MAKE(new_root));
 170
 171                 if (mount(".", "/", NULL, MS_MOVE, NULL) < 0)
 172                         return log_error_errno(errno, "Failed to move %s to /: %m", new_root);
 173
 174                 if (chroot(".") < 0)
 175                         return log_error_errno(errno, "Failed to change root: %m");
 176
 177                 if (chdir(".") < 0)
 178                         return log_error_errno(errno, "Failed to change directory: %m");
 179         }
 180
 181         if (istmp > 0) {
 182                 struct stat rb;
 183
 184                 if (fstat(old_root_fd, &rb) < 0)
 185                         return log_error_errno(errno, "Failed to stat old root directory: %m");
 186
 187                 /* Note: the below won't operate on non-memory file systems (i.e. only on tmpfs, ramfs), and
 188                  * it will stop at mount boundaries */
 189                 (void) rm_rf_children(TAKE_FD(old_root_fd), 0, &rb); /* takes possession of the dir fd, even on failure */
 190         }
 191
 192         return 0;
 193 }