-- A cgit auth filter that puts chosen repositories behind a login form and a
-- signed session cookie. cgit consults it on every request once cgitrc names
-- it with auth-filter=lua:/path/to/auth-inline.lua, and it answers the
-- authenticate-cookie, authenticate-post and body actions the filter API
-- defines. This variant carries the accounts and the per-repository access
-- lists in two tables inside the script, which suits a small fixed set of
-- users. The companion auth-file.lua behaves identically but reads the same
-- lists from files on disk.

-- The script runs on Lua 5.1 through 5.5 and LuaJIT, and the runtime has to be
-- the same Lua that cgit was built against. On 5.5 it needs luaossl 20260910
-- or later, since earlier builds fail to load there.
--
-- The session cookie is marked Secure by default, so a browser only sends
-- it back over HTTPS. On an instance served over plain HTTP login loops
-- until cookie_insecure below is set.
--
-- Two libraries are needed. luaossl provides openssl.rand and openssl.hmac and
-- lives at <https://github.com/wahern/luaossl>, and luaposix provides
-- posix.sys.stat and posix.unistd and lives at
-- <https://github.com/luaposix/luaposix>. The reliable cross-platform way to
-- install them is LuaRocks matched to your Lua version, and luaossl also needs
-- the OpenSSL development headers present. Each is loaded only once a request
-- needs it, so a public repository is served without them while a protected
-- one is refused.
--
--     # Debian and Ubuntu
--     sudo apt install luarocks libssl-dev
--     sudo luarocks --lua-version 5.1 install luaossl
--     sudo luarocks --lua-version 5.1 install luaposix
--
--     # Fedora
--     sudo dnf install luarocks openssl-devel
--     sudo luarocks --lua-version 5.1 install luaossl luaposix
--
--     # Alpine
--     sudo apk add luarocks openssl-dev
--     sudo luarocks-5.1 install luaossl luaposix
--
--     # macOS with Homebrew
--     brew install luarocks openssl
--     luarocks install luaossl OPENSSL_DIR="$(brew --prefix openssl)"
--     luarocks install luaposix
--
-- A session is signed together with its account's hash, so changing the
-- password or deleting the account ends it. There is no server-side session
-- store and no CSRF token, so instances sharing a secret file accept each
-- other's cookies, and only a new secret file ends every session at once.
-- That is acceptable for gating read access to a git browser, so weigh it
-- before guarding anything more.
--
-- The filter guards a repository's pages, not its place on the index, which
-- still lists a protected repository with its description unless cgitrc sets
-- repo.hide=1 for it.


-- Protected repositories and the users allowed into each. A repository named
-- here is protected and one that is not named is public. The repository key
-- has to match exactly, while user names match regardless of case. The
-- examples are commented out, so an unedited copy protects nothing.
local protected_repos = {
	-- ["secret-repo"] = { alice = true, bob = true },
	-- ["another"]     = { alice = true },
}

-- Accounts as name and hash, where the hash is any kind this system's crypt(3)
-- checks. yescrypt ($y$) is what libxcrypt recommends, as on Debian, Ubuntu and
-- Fedora, though musl, as on Alpine, does not check it. SHA-512 ($6$) works on
-- Linux and FreeBSD given enough rounds, bcrypt ($2b$) is the only kind OpenBSD
-- checks, and macOS checks none of them. Make one with whichever of these the
-- system has.
--
--     # Debian and Ubuntu, from the whois package
--     mkpasswd -m yescrypt
--     # anywhere with OpenSSL, as SHA-512
--     openssl passwd -6 -salt "rounds=300000\$$(openssl rand -hex 8)"
--     # OpenBSD, as bcrypt
--     encrypt -b a -p
local users = {
	-- alice = "$6$rounds=300000$REPLACE_THIS_SALT$REPLACE_THIS_HASH",
	-- bob   = "$6$rounds=300000$REPLACE_THIS_SALT$REPLACE_THIS_HASH",
}

-- Where the cookie-signing secret is stored, created on first use. It must be
-- persistent and writable by cgit, and it is worth keeping outside the cache
-- root, because pruning the cache would delete a secret kept inside it and
-- invalidate every live session. This file should not be world-readable.
local secret_filename = "/var/lib/cgit/auth-secret"

-- How long a login stays valid, in seconds. The default is one week.
local session_seconds = 7 * 24 * 60 * 60

local cookie_name = "cgitauth"

-- "/" covers the whole host. Set this to the cgit root to scope the cookie
-- more tightly.
local cookie_path = "/"

-- Set this true only if cgit is served over plain HTTP with no TLS anywhere.
local cookie_insecure = false

-- crypt feeds the password into most of its rounds, so its work grows with
-- the password's length, and one longer than this is turned down unchecked.
local max_password_bytes = 256


-- A browser accepts a cookie named __Host- only when it is Secure, covers the
-- whole host and was set by this host, so no sibling subdomain can plant a
-- session in its place. Either cookie setting above can rule that out.
local session_cookie = cookie_name
if not cookie_insecure and cookie_path == "/" then
	session_cookie = "__Host-" .. cookie_name
end

-- A cookie carries its expiry and Max-Age as whole seconds, while on Lua 5.3
-- and later a length worked out with a fraction, such as 1.5 days, is a float
-- that tostring would write with a trailing ".0".
session_seconds = math.floor(session_seconds)

-- One request reaches this script as an open, a write and a close, so what the
-- open decodes is kept here for the calls that follow.
local action, http, cgit, post

-- Apart from the configuration, the lookups below, account_hash and
-- repo_userset with the fold that feeds them, are the only code that
-- differs from auth-file.lua, which defines trim among its lookups.

-- The configured tables are folded to lowercased user names once, so that a
-- lookup matches whatever case the login form was filled in with, the way
-- auth-file.lua does. Repository names keep their case.
do
	local folded_users = {}
	for name, hash in pairs(users) do
		folded_users[tostring(name):lower()] = hash
	end
	users = folded_users
	for repo, members in pairs(protected_repos) do
		local folded = {}
		for name, allowed in pairs(members) do
			folded[tostring(name):lower()] = allowed
		end
		protected_repos[repo] = folded
	end
end

-- A name with no account comes back as nil and another account's hash, so
-- a failed login can spend the same work on a hash of the same kind and
-- cost, and its timing does not tell whether the name exists.
function account_hash(user)
	local hash = users[user:lower()]
	if hash ~= nil then
		return hash
	end
	local _, stand_in = next(users)
	return nil, stand_in
end

-- The users allowed into a repository come back keyed by lowercased name, and
-- a repository the table does not name comes back as nil.
function repo_userset(repo)
	return protected_repos[repo]
end

-- The url helpers below are adapted from keplerproject/wsapi.

function url_decode(str)
	str = string.gsub(str, "+", " ")
	str = string.gsub(str, "%%(%x%x)", function(hex)
		return string.char(tonumber(hex, 16))
	end)
	return str
end

function url_encode(str)
	str = string.gsub(str, "([^%w ])", function(char)
		return string.format("%%%02X", string.byte(char))
	end)
	str = string.gsub(str, " ", "+")
	return str
end

-- Parse an application/x-www-form-urlencoded body. A value may itself contain
-- '=', for example a base64 password, so the value runs to the next '&'.
function parse_query(query)
	local params = {}
	for key, value in string.gmatch(query, "([^&=]+)=([^&]*)") do
		params[url_decode(key)] = url_decode(value)
	end
	return params
end

-- Lua patterns backtrack, so "^%s*(.-)%s*$" costs the square of a run of
-- spaces inside the text. Finding each end directly stays linear, which
-- matters for the Cookie header a visitor writes.
local function trim(text)
	local first = text:find("%S")
	if first == nil then
		return ""
	end
	local last = #text
	while text:find("^%s", last) do
		last = last - 1
	end
	return text:sub(first, last)
end

-- The token comes back still url encoded by secure_value. Decoding it here
-- would break the signature check for any value carrying a percent escape.
function get_cookie(cookies, name)
	for raw in string.gmatch(cookies or "", "[^;]+") do
		local pair = trim(raw)
		local equals = pair:find("=", 1, true)
		if equals and pair:sub(1, equals - 1) == name then
			return pair:sub(equals + 1)
		end
	end
	return nil
end

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

local secret = nil

-- Load the cookie-signing secret, creating it on first use. Failures raise,
-- which cgit turns into a request error, so a broken secret denies rather than
-- signs with nothing.
function get_secret()
	if secret ~= nil then
		return secret
	end
	local secret_file = io.open(secret_filename, "r")
	if secret_file == nil then
		local sysstat = require("posix.sys.stat")
		local unistd = require("posix.unistd")
		local rand = require("openssl.rand")
		-- The secret is written under a tightened mask so it is not
		-- created readable by anyone but the user cgit runs as, and
		-- the old mask goes back on every way out.
		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
			sysstat.umask(old_umask)
			error("cgit auth: cannot create secret file " .. secret_filename)
		end
		local wrote = temporary_file:write(tohex(rand.bytes(32)))
		local closed = temporary_file:close()
		if not wrote or not closed then
			os.remove(temporary_filename)
			sysstat.umask(old_umask)
			error("cgit auth: failed writing secret file " .. secret_filename)
		end
		-- The link is meant to fail when another worker won the race,
		-- which leaves that worker's secret in place rather than
		-- replacing it and invalidating the sessions it just signed.
		unistd.link(temporary_filename, secret_filename)
		unistd.unlink(temporary_filename)
		sysstat.umask(old_umask)
		secret_file = io.open(secret_filename, "r")
	end
	if secret_file == nil then
		error("cgit auth: cannot read secret file " .. secret_filename)
	end
	secret = secret_file:read("*l")
	secret_file:close()
	if secret == nil or secret:len() ~= 64 then
		secret = nil
		error("cgit auth: secret file " .. secret_filename .. " is malformed, expected 64 hex characters")
	end
	return secret
end

-- A redirect target is unsafe if a browser would read it as another origin.
-- The only such form cgit can be tricked into signing is a scheme-relative
-- "//host" or "/\host". Everything else stays on this host.
function is_safe_redirect(url)
	if type(url) ~= "string" then
		return false
	end
	local head = url:sub(1, 2)
	if head == "//" or head == "/\\" then
		return false
	end
	return true
end

-- The signature covers the payload and a binding that the cookie never
-- carries, so a value only verifies while its binding stays the same.
local function sign(payload, binding)
	local hmac = require("openssl.hmac")
	local signed = payload .. "|" .. (binding or "")
	return tohex(hmac.new(get_secret(), "sha256"):final(signed))
end

-- Return the value carried by a signed cookie, or nil if it does not verify.
-- bind, when given, maps the value to the binding its signature has to cover,
-- or to nil to turn it down, which is how a session meets its account's
-- current hash.
function validate_value(expected_field, cookie, bind)
	local field, value, expiration, salt, signature =
		string.match(cookie or "", "^([^|]+)|([^|]+)|(%d+)|([^|]+)|([^|]+)$")
	if signature == nil or url_decode(field) ~= expected_field then
		return nil
	end

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

	local binding = ""
	if bind ~= nil then
		binding = bind(decoded)
		if binding == nil then
			return nil
		end
	end

	local payload = field .. "|" .. value .. "|" .. expiration .. "|" .. salt
	if not constant_equals(signature, sign(payload, binding)) then
		return nil
	end

	-- An expiration of 0 never expires and is used for the redirect token.
	expiration = tonumber(expiration)
	if expiration ~= 0 and expiration <= os.time() then
		return nil
	end
	return decoded
end

-- The layout built here is what validate_value takes apart again, so the two
-- have to move together. The binding, if any, is signed but left out.
function secure_value(field, value, expiration, binding)
	if value == nil or value:len() <= 0 then
		return ""
	end

	local rand = require("openssl.rand")
	local salt = tohex(rand.bytes(16))
	value = url_encode(value)
	field = url_encode(field)
	local payload = field .. "|" .. value .. "|" .. tostring(expiration) .. "|" .. salt
	return payload .. "|" .. sign(payload, binding)
end

-- Compare two strings in time that does not depend on how many leading bytes
-- match, so a mismatch position is not revealed by timing.
function constant_equals(a, b)
	if type(a) ~= "string" or type(b) ~= "string" or #a ~= #b then
		return false
	end
	local diff = 0
	for i = 1, #a do
		local delta = a:byte(i) - b:byte(i)
		diff = diff + delta * delta
	end
	return diff == 0
end

-- An empty value is how a login is cleared, since Max-Age=0 tells the browser
-- to drop the cookie it already holds. Any other value is a token secure_value
-- built, which holds no control character to split the response with.
function set_cookie(cookie, value)
	local attributes = "; HttpOnly; SameSite=Lax; Path=" .. cookie_path
	if not cookie_insecure then
		attributes = attributes .. "; Secure"
	end
	if value == "" then
		attributes = attributes .. "; Max-Age=0"
	else
		attributes = attributes .. "; Max-Age=" .. tostring(session_seconds)
	end
	html("Set-Cookie: " .. cookie .. "=" .. value .. attributes .. "\n")
end

-- A control character in the location would let it split the response, so
-- it is dropped rather than escaped.
function redirect_to(url)
	html("Status: 302 Found\n")
	html("Cache-Control: no-cache, no-store\n")
	html("Location: " .. url:gsub("%c", "") .. "\n")
end

-- A failed login lands back on the page it came from with this flag, which
-- the form reads to say so and leaves out of the way back it signs.
local failed_flag = "login=failed"

local function flag_failure(url)
	return url .. (url:find("?", 1, true) and "&" or "?") .. failed_flag
end

local function drop_failure_flag(url)
	return (url:gsub("[?&]" .. failed_flag .. "$", ""))
end

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

-- The three actions cgit can ask for follow, and they are the same in both
-- variants, so a change to one belongs in the other.

-- A wrong password and an unknown user answer alike, so the flag on the way
-- back says only that the login failed.
function authenticate_post()
	local redirect = validate_value("redirect", post["redirect"])

	if not is_safe_redirect(redirect) then
		not_found()
		return 0
	end

	local username = post["username"]
	local password = post["password"]
	local session = ""
	if username ~= nil and password ~= nil and #password <= max_password_bytes then
		local unistd = require("posix.unistd")
		local hash, stand_in = account_hash(username)
		if hash == nil then
			-- Spend the work anyway, see account_hash.
			if stand_in ~= nil then
				unistd.crypt(password, stand_in)
			end
		elseif constant_equals(hash, unistd.crypt(password, hash)) then
			session = secure_value("username", username, os.time() + session_seconds, hash)
		end
	end

	-- A failed login answers with an empty session, which clears the cookie.
	redirect_to(session == "" and flag_failure(redirect) or redirect)
	set_cookie(session_cookie, session)

	html("\n")
	return 0
end

-- cgit reads the answer, where 1 lets the request through and 0 sends it to
-- the login form.
function authenticate_cookie()
	local accepted_users = repo_userset(cgit["repo"])
	if accepted_users == nil then
		-- A repository nothing lists is public.
		return 1
	end

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

-- cgit calls this to fill the page body once the cookie has been turned down.
-- The form carries a signed redirect token, so the browser lands back on the
-- page that was asked for.
function body()
	local target = drop_failure_flag(cgit["url"])
	local failed = target ~= cgit["url"]
	if not is_safe_redirect(target) then
		target = cgit["login"]
	end

	html("<h2>Authentication Required</h2>")
	if failed then
		html("<p class='login-error' role='alert'>Wrong username or password.</p>")
	end
	html("<form class='login' method='post' action='")
	html_attr(cgit["login"])
	html("'>")
	html("<input type='hidden' name='redirect' value='")
	html_attr(secure_value("redirect", target, 0))
	html("'>")
	html("<table>")
	html("<tr><td><label for='username'>Username:</label></td><td>")
	html("<input id='username' name='username' type='text'")
	html(" autocomplete='username' autofocus></td></tr>")
	html("<tr><td><label for='password'>Password:</label></td><td>")
	html("<input id='password' name='password' type='password'")
	html(" autocomplete='current-password'></td></tr>")
	html("<tr><td></td><td>")
	html("<input value='Login' type='submit'></td></tr>")
	html("</table></form>")

	return 0
end

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

function filter_open(name, cookie, method, query, referer, path, host, https,
		repo, page, url, login)
	action = actions[name]
	post = {}
	http = { cookie = cookie }
	cgit = { repo = repo, url = url, login = login }
end

function filter_close()
	if action == nil then
		-- An unknown action denies rather than raising.
		return 0
	end
	return action()
end

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