LG G4 - Touchscreen Driver write_log Kernel Read/Write

Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=990

The following function (and variations on the same code) is used to write to 
files from kernel code in various touchscreen drivers. This copy is from 
RefCode_CustomerImplementation.c - I'm unsure which copy is actually being used
on the LG G4, but I can trigger the vulnerability. A function with the same
issues exists as "write_file" in several files.

int _write_log(char *filename, char *data)
{
  struct file *file;
  loff_t pos = 0;
  int flags;
  char *fname = "/data/logger/touch_self_test.txt";
  char *fname_normal_boot = "/sdcard/touch_self_test.txt";
  char *fname_mfts_folder = "/data/logger/touch_self_test_mfts_folder.txt";
  char *fname_mfts_flat = "/data/logger/touch_self_test_mfts_flat.txt";
  char *fname_mfts_curved = "/data/logger/touch_self_test_mfts_curved.txt";
  int cap_file_exist = 0;

  if (f54_window_crack || f54_window_crack_check_mode == 0) {
    mm_segment_t old_fs = get_fs();
    set_fs(KERNEL_DS);
    flags = O_WRONLY | O_CREAT;

    if (filename == NULL) {
      flags |= O_APPEND;
      switch (mfts_mode) {
      case 0:
        if (factory_boot)
          filename = fname;
        else
          filename = fname_normal_boot;
        break;
      case 1:
        filename = fname_mfts_folder;
        break;
      case 2:
        filename = fname_mfts_flat;
        break;
      case 3:
        filename = fname_mfts_curved;
        break;
      default:
        TOUCH_I("%s : not support mfts_mode\n",
          __func__);
        break;
      }
    } else {
      cap_file_exist = 1;
    }

    if (filename) {
      file = filp_open(filename, flags, 0666);
      sys_chmod(filename, 0666);
    } else {
      TOUCH_E("%s : filename is NULL, can not open FILE\n",
        __func__);
      return -1;
    }

    if (IS_ERR(file)) {
      TOUCH_I("%s : ERR(%ld)  Open file error [%s]\n",
          __func__, PTR_ERR(file), filename);
      set_fs(old_fs);
      return PTR_ERR(file);
    }

    vfs_write(file, data, strlen(data), &pos);
    filp_close(file, 0);
    set_fs(old_fs);

    log_file_size_check(filename);
  }
  return cap_file_exist;
}

int write_log(char *filename, char *data)
{
    return _write_log(filename, data);
}

This code is setting KERNEL_DS, and there is a code-path in which it does not 
restore USER_DS before returning (when mfts_mode is outside the range [0, 3] and
the filename argument is NULL). This can be triggered by first writing to the 
sysfs node /sys/devices/virtual/input/lge_touch/mfts and then reading from the 
sysfs node /sys/devices/virtual/input/lge_touch/sd. (root needed to write to mfts node).

Once the kernel has returned control to userland with KERNEL_DS set, userland can simply read/write from arbitrary kernel addresses. See 
attached for a working exploit for the LG G4, which when run as root will 
disable selinux enforcement.


Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/41353.zip