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MINIFILTER.md

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minifilter-rs

Use cargo doc --no-deps --document-private-items --open to read Documentation

Table of Contents

Table of Contents

Minifilter Driver

Building Driver

  1. Open VS 2022
  2. Goto minifilter-rs -> minifilter -> RWatch.sln
  3. Build the solution in Release mode with x64

NOTE: Enable Loading of Test Signed Drivers by executing Bcdedit.exe -set TESTSIGNING ON in administrative cmd

Installing Driver

  1. Open Powershell or command prompt as Administrator
  2. RUNDLL32.EXE SETUPAPI.DLL,InstallHinfSection DefaultInstall 132 <path-to>\minifilter-rs\minifilter\x64\Debug\snFilter.inf

You should be able to see the driver at "C:\Windows\System32\drivers\snFilter.sys"

Loading/Removing Driver

  1. Open Powershell or command prompt as Administrator
  2. Start the driver using sc start snFilter, expected output: 
    SERVICE_NAME: snFilter
    TYPE               : 2  FILE_SYSTEM_DRIVER
    STATE              : 4  RUNNING
                            (STOPPABLE, NOT_PAUSABLE, IGNORES_SHUTDOWN)
    WIN32_EXIT_CODE    : 0  (0x0)
    SERVICE_EXIT_CODE  : 0  (0x0)
    CHECKPOINT         : 0x0
    WAIT_HINT          : 0x0
    PID                : 0
    FLAGS              :
  3. Stop the driver using sc stop snFilter, should give the following output: 
    SERVICE_NAME: snFilter
    TYPE               : 2  FILE_SYSTEM_DRIVER
    STATE              : 1  STOPPED
    WIN32_EXIT_CODE    : 0  (0x0)
    SERVICE_EXIT_CODE  : 0  (0x0)
    CHECKPOINT         : 0x0
    WAIT_HINT          : 0x0
  4. Remove it by sc delete snFilter, should give the following output: 
    [SC] DeleteService SUCCESS

You can also run Fltmc.exe to see the currently loaded drivers:


Filter Name                     Num Instances    Altitude    Frame
------------------------------  -------------  ------------  -----
bindflt                                 1       409800         0
snFilter                                4       378781         0   // our minifilter driver
WdFilter                                5       328010         0
storqosflt                              0       244000         0
wcifs                                   0       189900         0
CldFlt                                  0       180451         0
FileCrypt                               0       141100         0
luafv                                   1       135000         0
npsvctrig                               1        46000         0
Wof                                     3        40700         0
FileInfo                                5        40500         0

Rust Application

Building Rust App

Simply use cargo build --release to build the application

Running Rust App

Use cargo run --bin minifilter --release to run the application

The program starts to print the IOMessage which is defined like:

#[repr(C)]
pub struct IOMessage {
    /// The file extension
    pub extension: [wchar_t; 12],
    /// Hard Disk Volume Serial Number where the file is saved (from [`FILE_ID_INFO`])
    pub file_id_vsn: c_ulonglong,
    /// File ID on the disk ([`FILE_ID_INFO`])
    pub file_id_id: [u8; 16],
    /// Number of bytes transferred (`IO_STATUS_BLOCK.Information`)
    pub mem_sized_used: c_ulonglong,
    /// (Optional) File Entropy calculated by the driver
    pub entropy: f64,
    /// Pid responsible for this io activity
    pub pid: c_ulong,
    /// Windows IRP Type caught by the minifilter:
    /// - NONE (0)
    /// - READ (1)
    /// - WRITE (2)
    /// - SETINFO (3)
    /// - CREATE (4)
    /// - CLEANUP (5)
    pub irp_op: c_uchar,
    /// Is the entropy calculated?
    pub is_entropy_calc: u8,
    /// Type of i/o operation:
    /// - FILE_CHANGE_NOT_SET (0)
    /// - FILE_OPEN_DIRECTORY (1)
    /// - FILE_CHANGE_WRITE (2)
    /// - FILE_CHANGE_NEW_FILE (3)
    /// - FILE_CHANGE_RENAME_FILE (4)
    /// - FILE_CHANGE_EXTENSION_CHANGED (5)
    /// - FILE_CHANGE_DELETE_FILE (6)
    /// - FILE_CHANGE_DELETE_NEW_FILE (7)
    /// - FILE_CHANGE_OVERWRITE_FILE (8)
    pub file_change: c_uchar,
    /// The driver has the ability to monitor specific directories only (feature currently not used):
    /// - FILE_NOT_PROTECTED (0): Monitored dirs do not contained this file
    /// - FILE_PROTECTED (1)
    /// - FILE_MOVED_IN (2)
    /// - FILE_MOVED_OUT (3)
    pub file_location_info: c_uchar,
    /// File path on the disk
    pub filepathstr: String,
    /// Group Identifier (maintained by the minifilter) of the operation
    pub gid: c_ulonglong,
    /// see class [`RuntimeFeatures`]
    pub runtime_features: RuntimeFeatures,
    /// Size of the file. Can be equal to -1 if the file path is not found.
    pub file_size: i64,
    /// Rough time at which the IRP was created
    pub time: SystemTime,
}

We end the process using ctrl + c in the example video: video

NOTE:

  • Might fail if not run with administrative privileges
  • You need to load and start the driver before running the program or else it will error out

What and the How

We basically share definition between the mini-filter and Rust using #[repr(C)]

shared_def

We use channels to process all IRPs.