-- cgit auth-filter that gates repositories behind a login form and a signed
-- session cookie.
--
-- This is the INLINE variant. The user accounts and the per-repository access
-- lists are written directly in this script, in the two tables in the
-- CONFIGURATION block below. Edit them here and reload. This suits a small
-- fixed set of users that rarely changes.
--
-- The companion auth-file.lua behaves identically but reads its accounts and
-- access lists from files on disk instead, which suits larger or externally
-- managed user sets.
--
-- Enable it in cgitrc with
--     auth-filter=lua:/path/to/auth-inline.lua
--
-- SUPPORTED LUA
--
-- Lua 5.1, 5.2, 5.3, 5.4 and LuaJIT. Lua 5.5 is not supported, because luaossl
-- has no 5.5 build. Match the runtime to the Lua that cgit is built against.
--
-- HTTPS IS RECOMMENDED
--
-- Serve cgit over HTTPS. Terminate TLS at the web server in front of cgit. The
-- session cookie is marked Secure by default, so a browser only sends it back
-- over HTTPS. If you genuinely run cgit over plain HTTP with no TLS anywhere,
-- set cookie_insecure below, otherwise the cookie is never returned and login
-- appears to loop.
--
-- DEPENDENCIES
--
--     luaossl    OpenSSL binding, provides openssl.rand and openssl.hmac
--                <https://github.com/wahern/luaossl>
--     luaposix   POSIX binding, provides posix.sys.stat and posix.unistd
--                <https://github.com/luaposix/luaposix>
--
-- The reliable cross-platform install is LuaRocks, matched to your Lua
-- version. luaossl also needs the OpenSSL development headers present.
--
--     # 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
--
-- Some distributions also package these, for example lua-luaossl and lua-posix
-- on Debian. If you use a distribution package, make sure it is built for the
-- same Lua version as cgit.
--
-- SECURITY NOTES
--
-- The cookie carries only a username with no server-side session store, so
-- deleting an account does not revoke a cookie already issued until it
-- expires, and instances that share a secret file accept each other's cookies.
-- The login form carries no CSRF token. Both are acceptable for gating read
-- access to a git browser. Weigh them before guarding anything more sensitive.

local sysstat = require("posix.sys.stat")
local unistd = require("posix.unistd")
local rand = require("openssl.rand")
local hmac = require("openssl.hmac")

--
-- ========================= CONFIGURATION =========================
-- Edit the values in this block. Nothing below it needs changing for
-- ordinary use.
--

-- Protected repositories and the users allowed into each. A repository listed
-- here is protected. One not listed is public. Keys and user names are matched
-- case-insensitively. REPLACE THE EXAMPLES BELOW, they are commented out so an
-- unedited copy protects nothing and grants no accounts.
local protected_repos = {
	-- ["secret-repo"] = { alice = true, bob = true },
	-- ["another"]     = { alice = true },
}

-- Accounts as name = hash. Generate a hash with
--     mkpasswd -m sha-512 -R 300000
-- REPLACE THE EXAMPLES BELOW. The commented lines are not real credentials and
-- must not be deployed as-is.
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. It is created on first use. It
-- must be persistent and writable by cgit. Prefer a path 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/cache/cgit/auth-secret"

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

-- Name of the session cookie.
local cookie_name = "cgitauth"

-- Path the cookie is scoped to. "/" covers the whole host. Set it to the cgit
-- root to scope the cookie more tightly.
local cookie_path = "/"

-- Leave false so the cookie is marked Secure and only travels over HTTPS. Set
-- it true ONLY if cgit is served over plain HTTP with no TLS anywhere, see the
-- HTTPS note in the header.
local cookie_insecure = false

--
-- =================================================================
--

-- A throwaway hash of the documented shape, used only to spend the same work
-- on a missing account as on a present one, so a failed login does not reveal
-- by timing whether the username exists.
local dummy_hash = "$6$rounds=300000$0000000000000000$"

-- Module state shared across the open, write and close calls of one request.
local action, http, cgit, post

--
--
-- Account and access-list storage. This is the ONLY part that differs from
-- auth-file.lua. Swap these two functions to change where accounts live.
--
--

-- Fold the configured tables to lowercased user names once, so lookups match
-- case-insensitively the same way auth-file.lua does. Repository names keep
-- their case.
do
	local folded = {}
	for name, hash in pairs(users) do
		folded[tostring(name):lower()] = hash
	end
	users = folded
	for repo, set in pairs(protected_repos) do
		local fs = {}
		for name, allowed in pairs(set) do
			fs[tostring(name):lower()] = allowed
		end
		protected_repos[repo] = fs
	end
end

-- Return the stored password hash for a user, or nil.
function account_hash(user)
	if user == nil then
		return nil
	end
	return users[user:lower()]
end

-- Return the set of users allowed to access a repository, keyed by lowercased
-- user name, or nil if the repository is not protected.
function repo_userset(repo)
	if repo == nil then
		return nil
	end
	return protected_repos[repo]
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

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

-- Return the value of the named cookie, or nil. The stored token was already
-- url-encoded by secure_value, so it is returned verbatim, which keeps the
-- write path (set_cookie) and the read path symmetric. Decoding it here would
-- break the signature check for any value carrying a percent escape.
function get_cookie(cookies, name)
	cookies = string.gsub(";" .. (cookies or "") .. ";", "%s*;%s*", ";")
	return 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

-- 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 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
		unistd.link(temporary_filename, secret_filename) -- Intentionally fails if another worker won the race.
		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

-- Return the value carried by a signed cookie, or nil if it does not verify.
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
			-- The expiration must be a plain integer. Rejecting other forms
			-- keeps the signed bytes canonical, since tonumber and tostring of
			-- "1e9" or "100.0" differ across Lua versions.
			if not string.match(component, "^%d+$") then
				return nil
			end
			expiration = tonumber(component)
		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

	-- An expiration of 0 never expires and is used for the redirect token.
	if 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 value 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 salt = tohex(rand.bytes(16))
	value = url_encode(value)
	field = url_encode(field)
	local 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 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 d = a:byte(i) - b:byte(i)
		diff = diff + d * d
	end
	return diff == 0
end

function set_cookie(cookie, value)
	local attrs = "; HttpOnly; SameSite=Lax; Path=" .. cookie_path
	if not cookie_insecure then
		attrs = attrs .. "; Secure"
	end
	if value == "" then
		attrs = attrs .. "; Max-Age=0"
	elseif session_seconds > 0 then
		attrs = attrs .. "; Max-Age=" .. tostring(session_seconds)
	end
	html("Set-Cookie: " .. cookie .. "=" .. strip_ctl(value) .. attrs .. "\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

--
--
-- Authentication actions. Identical to auth-inline.lua from here down.
--
--

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

	if redirect == nil or not is_safe_redirect(redirect) then
		not_found()
		return 0
	end

	redirect_to(redirect)

	local username = post["username"]
	local password = post["password"]
	local ok = false
	if username ~= nil and password ~= nil then
		local hash = account_hash(username)
		if hash == nil then
			-- Spend the work anyway, see dummy_hash.
			unistd.crypt(password, dummy_hash)
		elseif constant_equals(hash, unistd.crypt(password, hash)) then
			ok = true
		end
	end

	if ok then
		set_cookie(cookie_name, secure_value("username", username, os.time() + session_seconds))
	else
		set_cookie(cookie_name, "")
	end

	html("\n")
	return 0
end

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

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

-- Prints the html for the login form.
function body()
	local target = cgit["url"]
	if not is_safe_redirect(target) then
		target = cgit["login"]
	end

	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", target, 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 the filter API, exposing the http, cgit and post tables to
-- the functions above.
--
--

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

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

	post = {}

	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()
	if action == nil then
		-- Unknown action, deny rather than raise.
		return 0
	end
	return action()
end

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