sudowrite.lua

local M = {}

local uv = vim.loop
local fn = vim.fn
local api = vim.api

function password()
  fn.inputsave()
  local user = fn.expand("$USER")
  local pw = fn.inputsecret(string.format("password for %s: ", user))
  fn.inputrestore()
  return pw
end

function test(pw, k)
  local stdin = uv.new_pipe()
  uv.spawn("sudo", {
    args = {"-S", "-k", "true"},
    stdio = {stdin, nil, nil}
  }, k)

  stdin:write(pw)
  stdin:write("\n")
  stdin:shutdown()
end

function write(pw, buf, lines, k)
  local stdin = uv.new_pipe()
  uv.spawn("sudo", {
    args = {"-S", "-k", "tee", buf},
    stdio = {stdin, nil, nil}
  }, k)

  stdin:write(pw)
  stdin:write("\n")
  local last = table.remove(lines)
  for _, line in ipairs(lines) do
    stdin:write(line)
    stdin:write("\n")
  end
  stdin:write(last)
  stdin:shutdown()
end

function M.sudowrite()
  local pw = password()
  local buf = api.nvim_buf_get_name(0)
  local lines = api.nvim_buf_get_lines(bufnr, 0, -1, false) 

  local function exitWrite(code, _)
    if code == 0 then
      vim.schedule(function()
        api.nvim_echo({{string.format('"%s" written', buf), "Normal"}}, true, {})
        api.nvim_buf_set_option(0, "modified", false)
      end)
    end 
  end
  local function exitTest(code, _)
    if code == 0 then
      write(pw, buf, lines, exitWrite)
    else
      vim.schedule(function()
        api.nvim_echo({{"incorrect password", "ErrorMsg"}}, true, {})
      end)
    end
  end
  test(pw, exitTest)
end

return M

Oops seems like it was issue from side of pam-fprint-grosshack, I'll check if anything there is preventing from using sudo -S. Thanks for the script, its working :)