index : cgit

-- cgit auth-filter that gates repositories behind a login form and a
-- signed session cookie.
--
-- This is the FILE-BACKED variant. The user accounts, the groups, and
-- the per-repository access lists are read from files on disk. Set their
-- paths below and edit those files without touching this script. This
-- suits larger or externally managed user sets.
--
-- The companion auth-inline.lua behaves identically but keeps its
-- accounts and access lists inline in the script itself, which suits a
-- small fixed set of users.
--
-- Enable it in cgitrc with
-- 	auth-filter=lua:/path/to/auth-file.lua
--
-- Requirements
-- 	luaossl
-- 	<http://25thandclement.com/~william/projects/luaossl.html>
-- 	luaposix
-- 	<https://github.com/luaposix/luaposix>
--
local sysstat = require("posix.sys.stat")
local unistd = require("posix.unistd")
local rand = require("openssl.rand")
local hmac = require("openssl.hmac")

-- This file should contain a series of lines in the form of:
--	username1:hash1
--	username2:hash2
--	username3:hash3
--	...
-- Hashes can be generated using something like `mkpasswd -m sha-512 -R 300000`.
-- This file should not be world-readable.
local users_filename = "/etc/cgit-auth/users"

-- This file should contain a series of lines in the form of:
-- 	groupname1:username1,username2,username3,...
--	...
local groups_filename = "/etc/cgit-auth/groups"

-- This file should contain a series of lines in the form of:
-- 	reponame1:groupname1,groupname2,groupname3,...
--	...
local repos_filename = "/etc/cgit-auth/repos"

-- Set this to a path this script can write to for storing a persistent
-- cookie secret, which should not be world-readable.
local secret_filename = "/var/cache/cgit/auth-secret"

--
--
-- Authentication functions follow below. Swap these out if you want different authentication semantics.
--
--

-- Looks up a hash for a given user.
function lookup_hash(user)
	local line
	for line in io.lines(users_filename) do
		local u, h = string.match(line, "(.-):(.+)")
		if u:lower() == user:lower() then
			return h
		end
	end
	return nil
end

-- Looks up users for a given repo.
function lookup_users(repo)
	local users = nil
	local groups = nil
	local line, group, user
	for line in io.lines(repos_filename) do
		local r, g = string.match(line, "(.-):(.+)")
		if r == repo then
			groups = { }
			for group in string.gmatch(g, "([^,]+)") do
				groups[group:lower()] = true
			end
			break
		end
	end
	if groups == nil then
		return nil
	end
	for line in io.lines(groups_filename) do
		local g, u = string.match(line, "(.-):(.+)")
		if groups[g:lower()] then
			if users == nil then
				users = { }
			end
			for user in string.gmatch(u, "([^,]+)") do
				users[user:lower()] = true
			end
		end
	end
	return users
end


-- Sets HTTP cookie headers based on post and sets up redirection.
function authenticate_post()
	local hash = lookup_hash(post["username"])
	local redirect = validate_value("redirect", post["redirect"])

	if redirect == nil then
		not_found()
		return 0
	end

	redirect_to(redirect)

	if hash == nil or hash ~= unistd.crypt(post["password"], hash) then
		set_cookie("cgitauth", "")
	else
		-- One week expiration time
		local username = secure_value("username", post["username"], os.time() + 604800)
		set_cookie("cgitauth", username)
	end

	html("\n")
	return 0
end


-- Returns 1 if the cookie is valid and 0 if it is not.
function authenticate_cookie()
	accepted_users = lookup_users(cgit["repo"])
	if accepted_users == nil then
		-- We return as valid if the repo is not protected.
		return 1
	end

	local username = validate_value("username", get_cookie(http["cookie"], "cgitauth"))
	if username == nil or not accepted_users[username:lower()] then
		return 0
	else
		return 1
	end
end

-- Prints the html for the login form.
function body()
	html("<h2>Authentication Required</h2>")
	html("<form method='post' action='")
	html_attr(cgit["login"])
	html("'>")
	html("<input type='hidden' name='redirect' value='")
	html_attr(secure_value("redirect", cgit["url"], 0))
	html("' />")
	html("<table>")
	html("<tr><td><label for='username'>Username:</label></td><td><input id='username' name='username' autofocus /></td></tr>")
	html("<tr><td><label for='password'>Password:</label></td><td><input id='password' name='password' type='password' /></td></tr>")
	html("<tr><td colspan='2'><input value='Login' type='submit' /></td></tr>")
	html("</table></form>")

	return 0
end



--
--
-- Wrapper around filter API, exposing the http table, the cgit table, and the post table to the above functions.
--
--

local actions = {}
actions["authenticate-post"] = authenticate_post
actions["authenticate-cookie"] = authenticate_cookie
actions["body"] = body

function filter_open(...)
	action = actions[select(1, ...)]

	http = {}
	http["cookie"] = select(2, ...)
	http["method"] = select(3, ...)
	http["query"] = select(4, ...)
	http["referer"] = select(5, ...)
	http["path"] = select(6, ...)
	http["host"] = select(7, ...)
	http["https"] = select(8, ...)

	cgit = {}
	cgit["repo"] = select(9, ...)
	cgit["page"] = select(10, ...)
	cgit["url"] = select(11, ...)
	cgit["login"] = select(12, ...)

end

function filter_close()
	return action()
end

function filter_write(str)
	post = parse_qs(str)
end


--
--
-- Utility functions based on keplerproject/wsapi.
--
--

function url_decode(str)
	if not str then
		return ""
	end
	str = string.gsub(str, "+", " ")
	str = string.gsub(str, "%%(%x%x)", function(h) return string.char(tonumber(h, 16)) end)
	str = string.gsub(str, "\r\n", "\n")
	return str
end

function url_encode(str)
	if not str then
		return ""
	end
	str = string.gsub(str, "\n", "\r\n")
	str = string.gsub(str, "([^%w ])", function(c) return string.format("%%%02X", string.byte(c)) end)
	str = string.gsub(str, " ", "+")
	return str
end

function parse_qs(qs)
	local tab = {}
	for key, val in string.gmatch(qs, "([^&=]+)=([^&=]*)&?") do
		tab[url_decode(key)] = url_decode(val)
	end
	return tab
end

function get_cookie(cookies, name)
	cookies = string.gsub(";" .. cookies .. ";", "%s*;%s*", ";")
	return url_decode(string.match(cookies, ";" .. name .. "=(.-);"))
end

function tohex(b)
	local x = ""
	for i = 1, #b do
		x = x .. string.format("%.2x", string.byte(b, i))
	end
	return x
end

--
--
-- Cookie construction and validation helpers.
--
--

local secret = nil

-- Loads a secret from a file, creates a secret, or returns one from memory.
function get_secret()
	if secret ~= nil then
		return secret
	end
	local secret_file = io.open(secret_filename, "r")
	if secret_file == nil then
		local old_umask = sysstat.umask(63)
		local temporary_filename = secret_filename .. ".tmp." .. tohex(rand.bytes(16))
		local temporary_file = io.open(temporary_filename, "w")
		if temporary_file == nil then
			os.exit(177)
		end
		temporary_file:write(tohex(rand.bytes(32)))
		temporary_file:close()
		unistd.link(temporary_filename, secret_filename) -- Intentionally fails in the case that another process is doing the same.
		unistd.unlink(temporary_filename)
		sysstat.umask(old_umask)
		secret_file = io.open(secret_filename, "r")
	end
	if secret_file == nil then
		os.exit(177)
	end
	secret = secret_file:read()
	secret_file:close()
	if secret:len() ~= 64 then
		os.exit(177)
	end
	return secret
end

-- Returns value of cookie if cookie is valid. Otherwise returns nil.
function validate_value(expected_field, cookie)
	local i = 0
	local value = ""
	local field = ""
	local expiration = 0
	local salt = ""
	local chmac = ""

	if cookie == nil or cookie:len() < 3 or cookie:sub(1, 1) == "|" then
		return nil
	end

	for component in string.gmatch(cookie, "[^|]+") do
		if i == 0 then
			field = component
		elseif i == 1 then
			value = component
		elseif i == 2 then
			expiration = tonumber(component)
			if expiration == nil then
				expiration = -1
			end
		elseif i == 3 then
			salt = component
		elseif i == 4 then
			chmac = component
		else
			break
		end
		i = i + 1
	end

	if chmac == nil or chmac:len() == 0 then
		return nil
	end

	-- Compare the HMAC without short-circuiting on the first mismatch.
	if not constant_equals(chmac, tohex(hmac.new(get_secret(), "sha256"):final(field .. "|" .. value .. "|" .. tostring(expiration) .. "|" .. salt))) then
		return nil
	end

	if expiration == -1 or (expiration ~= 0 and expiration <= os.time()) then
		return nil
	end

	if url_decode(field) ~= expected_field then
		return nil
	end

	local decoded = url_decode(value)
	-- Reject values carrying control characters so a signed cookie or
	-- redirect target cannot smuggle CR or LF into a response header.
	if decoded:find("%c") then
		return nil
	end
	return decoded
end

function secure_value(field, value, expiration)
	if value == nil or value:len() <= 0 then
		return ""
	end

	local authstr = ""
	local salt = tohex(rand.bytes(16))
	value = url_encode(value)
	field = url_encode(field)
	authstr = field .. "|" .. value .. "|" .. tostring(expiration) .. "|" .. salt
	authstr = authstr .. "|" .. tohex(hmac.new(get_secret(), "sha256"):final(authstr))
	return authstr
end

-- Strip control characters that could split an HTTP response header.
function strip_ctl(s)
	return (string.gsub(s or "", "%c", ""))
end

-- Compare two strings without stopping at the first mismatch, so the time
-- taken does not reveal how many leading bytes already matched.
function constant_equals(a, b)
	if a == nil or b == nil or #a ~= #b then
		return false
	end
	local diff = 0
	for i = 1, #a do
		if a:byte(i) ~= b:byte(i) then
			diff = diff + 1
		end
	end
	return diff == 0
end

function set_cookie(cookie, value)
	html("Set-Cookie: " .. cookie .. "=" .. strip_ctl(value) .. "; HttpOnly; SameSite=Lax")
	if http["https"] == "yes" or http["https"] == "on" or http["https"] == "1" then
		html("; secure")
	end
	html("\n")
end

function redirect_to(url)
	html("Status: 302 Redirect\n")
	html("Cache-Control: no-cache, no-store\n")
	html("Location: " .. strip_ctl(url) .. "\n")
end

function not_found()
	html("Status: 404 Not Found\n")
	html("Cache-Control: no-cache, no-store\n\n")
end