platform_system_sepolicy/public/fsck_untrusted.te

# Any fsck program run on untrusted block devices
type fsck_untrusted, domain;

# Inherit and use pty created by android_fork_execvp_ext().
allow fsck_untrusted devpts:chr_file { read write ioctl getattr };

# Allow stdin/out back to vold
allow fsck_untrusted vold:fd use;
allow fsck_untrusted vold:fifo_file { read write getattr };

# Run fsck on vold block devices
allow fsck_untrusted block_device:dir search;
allow fsck_untrusted vold_device:blk_file rw_file_perms;

allow fsck_untrusted proc_mounts:file r_file_perms;

# To determine if it is safe to run fsck on a filesystem, e2fsck
# must first determine if the filesystem is mounted. To do that,
# e2fsck scans through /proc/mounts and collects all the mounted
# block devices. With that information, it runs stat() on each block
# device, comparing the major and minor numbers to the filesystem
# passed in on the command line. If there is a match, then the filesystem
# is currently mounted and running fsck is dangerous.
# Allow stat access to all block devices so that fsck can compare
# major/minor values.
allow fsck_untrusted dev_type:blk_file getattr;

###
### neverallow rules
###

# Untrusted fsck should never be run on block devices holding sensitive data
neverallow fsck_untrusted {
  boot_block_device
  frp_block_device
  metadata_block_device
  recovery_block_device
  root_block_device
  swap_block_device
  system_block_device
  userdata_block_device
  cache_block_device
  dm_device
}:blk_file no_rw_file_perms;

# Only allow entry from vold via fsck binaries
neverallow { domain -vold } fsck_untrusted:process transition;
neverallow * fsck_untrusted:process dyntransition;
neverallow fsck_untrusted { file_type fs_type -fsck_exec }:file entrypoint;