moonlock/src/auth.rs

// ABOUTME: PAM authentication via raw FFI wrapper around libpam.so.
// ABOUTME: Provides authenticate(username, password) for the lockscreen with secure password wiping.

use std::ffi::CString;
use std::ptr;
use zeroize::Zeroizing;

// PAM return codes
const PAM_SUCCESS: i32 = 0;
const PAM_BUF_ERR: i32 = 5;

// PAM message styles
const PAM_PROMPT_ECHO_OFF: libc::c_int = 1;
const PAM_PROMPT_ECHO_ON: libc::c_int = 2;

/// PAM message structure (pam_message).
#[repr(C)]
struct PamMessage {
    msg_style: libc::c_int,
    msg: *const libc::c_char,
}

/// PAM response structure (pam_response).
#[repr(C)]
struct PamResponse {
    resp: *mut libc::c_char,
    resp_retcode: libc::c_int,
}

/// PAM conversation callback type.
type PamConvFn = unsafe extern "C" fn(
    num_msg: libc::c_int,
    msg: *mut *const PamMessage,
    resp: *mut *mut PamResponse,
    appdata_ptr: *mut libc::c_void,
) -> libc::c_int;

/// PAM conversation structure (pam_conv).
#[repr(C)]
struct PamConv {
    conv: PamConvFn,
    appdata_ptr: *mut libc::c_void,
}

// libpam function declarations
unsafe extern "C" {
    fn pam_start(
        service_name: *const libc::c_char,
        user: *const libc::c_char,
        pam_conversation: *const PamConv,
        pamh: *mut *mut libc::c_void,
    ) -> libc::c_int;

    fn pam_authenticate(pamh: *mut libc::c_void, flags: libc::c_int) -> libc::c_int;

    fn pam_acct_mgmt(pamh: *mut libc::c_void, flags: libc::c_int) -> libc::c_int;

    fn pam_end(pamh: *mut libc::c_void, pam_status: libc::c_int) -> libc::c_int;
}

/// PAM conversation callback — provides the password to PAM.
///
/// # Safety
/// Called by libpam during authentication. Allocates response memory with calloc/strdup
/// which PAM will free. The appdata_ptr must point to a valid CString (the password).
unsafe extern "C" fn pam_conv_callback(
    num_msg: libc::c_int,
    msg: *mut *const PamMessage,
    resp: *mut *mut PamResponse,
    appdata_ptr: *mut libc::c_void,
) -> libc::c_int {
    unsafe {
        // Safety: appdata_ptr was set to a valid *const CString in authenticate()
        let password = appdata_ptr as *const CString;
        if password.is_null() {
            return 7; // PAM_AUTH_ERR
        }

        // Safety: calloc returns zeroed memory for num_msg PamResponse structs.
        // PAM owns this memory and will free() it.
        let resp_array = libc::calloc(
            num_msg as libc::size_t,
            std::mem::size_of::<PamResponse>() as libc::size_t,
        ) as *mut PamResponse;

        if resp_array.is_null() {
            return 7; // PAM_AUTH_ERR
        }

        for i in 0..num_msg as isize {
            let resp_ptr = resp_array.offset(i);
            // Safety: msg is an array of pointers provided by PAM
            let pam_msg = *msg.offset(i);
            let msg_style = (*pam_msg).msg_style;

            match msg_style {
                PAM_PROMPT_ECHO_OFF => {
                    // Password prompt — provide the password via strdup
                    let dup = libc::strdup((*password).as_ptr());
                    if dup.is_null() {
                        // strdup failed (OOM) — free all previously allocated strings
                        for j in 0..i {
                            let prev = resp_array.offset(j);
                            if !(*prev).resp.is_null() {
                                libc::free((*prev).resp as *mut libc::c_void);
                            }
                        }
                        libc::free(resp_array as *mut libc::c_void);
                        return PAM_BUF_ERR;
                    }
                    (*resp_ptr).resp = dup;
                }
                PAM_PROMPT_ECHO_ON => {
                    // Visible prompt — provide empty string, never the password
                    let empty = libc::strdup(b"\0".as_ptr() as *const libc::c_char);
                    if empty.is_null() {
                        for j in 0..i {
                            let prev = resp_array.offset(j);
                            if !(*prev).resp.is_null() {
                                libc::free((*prev).resp as *mut libc::c_void);
                            }
                        }
                        libc::free(resp_array as *mut libc::c_void);
                        return PAM_BUF_ERR;
                    }
                    (*resp_ptr).resp = empty;
                }
                _ => {
                    // PAM_ERROR_MSG, PAM_TEXT_INFO, or unknown — no response expected
                    (*resp_ptr).resp = ptr::null_mut();
                }
            }
            (*resp_ptr).resp_retcode = 0;
        }

        // Safety: resp is a valid pointer provided by PAM
        *resp = resp_array;
        PAM_SUCCESS
    }
}

/// Authenticate a user via PAM.
///
/// Returns true on success, false on authentication failure.
/// The password is wiped from memory after use via zeroize.
pub fn authenticate(username: &str, password: &str) -> bool {
    // Use Zeroizing to ensure password bytes are wiped on drop
    let password_bytes = Zeroizing::new(password.as_bytes().to_vec());
    let password_cstr = match CString::new(password_bytes.as_slice()) {
        Ok(c) => c,
        Err(_) => return false, // Password contains null byte
    };

    let service = match CString::new("moonlock") {
        Ok(c) => c,
        Err(_) => return false,
    };

    let username_cstr = match CString::new(username) {
        Ok(c) => c,
        Err(_) => return false,
    };

    let conv = PamConv {
        conv: pam_conv_callback,
        appdata_ptr: &password_cstr as *const CString as *mut libc::c_void,
    };

    let mut handle: *mut libc::c_void = ptr::null_mut();

    // Safety: All pointers are valid CStrings that outlive the pam_start call.
    // handle is an output parameter that PAM will set.
    let ret = unsafe {
        pam_start(
            service.as_ptr(),
            username_cstr.as_ptr(),
            &conv,
            &mut handle,
        )
    };

    if ret != PAM_SUCCESS {
        return false;
    }

    if handle.is_null() {
        return false;
    }

    // Safety: handle is valid and non-null after successful pam_start
    let auth_ret = unsafe { pam_authenticate(handle, 0) };
    let acct_ret = if auth_ret == PAM_SUCCESS {
        // Safety: handle is valid, check account restrictions
        unsafe { pam_acct_mgmt(handle, 0) }
    } else {
        auth_ret
    };

    // Safety: handle is valid, pam_end cleans up the PAM session
    unsafe { pam_end(handle, acct_ret) };

    acct_ret == PAM_SUCCESS
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn pam_service_name_is_valid_cstring() {
        let service = CString::new("moonlock").unwrap();
        assert_eq!(service.as_bytes(), b"moonlock");
    }

    #[test]
    fn password_with_null_byte_fails() {
        // authenticate should return false for passwords with embedded nulls
        let result = authenticate("testuser", "pass\0word");
        assert!(!result);
    }

    #[test]
    fn zeroizing_wipes_password() {
        let password = Zeroizing::new(vec![0x41u8, 0x42, 0x43]);
        let ptr = password.as_ptr();
        drop(password);
        // After drop, the memory should be zeroed (though we can't reliably
        // test this since the allocator may reuse the memory). This test
        // verifies the Zeroizing wrapper compiles and drops correctly.
        assert!(!ptr.is_null());
    }

    #[test]
    fn empty_username_fails() {
        // Empty username should not crash, just fail auth
        let result = authenticate("", "password");
        assert!(!result);
    }
}