TraversalLimit.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}

-- |
-- Module: Capnp.TraversalLimit
-- Description: Support for managing message traversal limits.
--
-- This module is used to mitigate several pitfalls with the capnproto format,
-- which could potentially lead to denial of service vulnerabilities.
--
-- In particular, while they are illegal according to the spec, it is possible to
-- encode objects which have many pointers pointing the same place, or even
-- cycles. A naive traversal therefore could involve quite a lot of computation
-- for a message that is very small on the wire.
--
-- Accordingly, most implementations of the format keep track of how many bytes
-- of a message have been accessed, and start signaling errors after a certain
-- value (the "traversal limit") has been reached. The Haskell implementation is
-- no exception; this module implements that logic. We provide a monad
-- transformer and mtl-style type class to track the limit; reading from the
-- message happens inside of this monad.
module Capnp.TraversalLimit
  ( MonadLimit (..),
    LimitT,
    runLimitT,
    evalLimitT,
    execLimitT,
    defaultLimit,
  )
where

-- Just to define 'MonadLimit' instances:

import Capnp.Bits (WordCount)
import Capnp.Errors (Error (TraversalLimitError))
import Control.Monad (when)
import Control.Monad.Catch (MonadCatch (catch), MonadThrow (throwM))
import Control.Monad.Fail (MonadFail (..))
import Control.Monad.IO.Class (MonadIO (..))
import Control.Monad.Primitive (PrimMonad (primitive), PrimState)
import Control.Monad.RWS (RWST)
import Control.Monad.Reader (ReaderT)
import qualified Control.Monad.State.Lazy as LazyState
import Control.Monad.State.Strict
  ( MonadState,
    StateT,
    evalStateT,
    execStateT,
    get,
    put,
    runStateT,
  )
import Control.Monad.Trans.Class (MonadTrans (lift))
import Control.Monad.Writer (WriterT)
import Prelude hiding (fail)

-- | mtl-style type class to track the traversal limit. This is used
-- by other parts of the library which actually do the reading.
class Monad m => MonadLimit m where
  -- | @'invoice' n@ deducts @n@ from the traversal limit, signaling
  -- an error if the limit is exhausted.
  invoice :: WordCount -> m ()

-- | Monad transformer implementing 'MonadLimit'. The underlying monad
-- must implement 'MonadThrow'. 'invoice' calls @'throwM' 'TraversalLimitError'@
-- when the limit is exhausted.
newtype LimitT m a = LimitT (StateT WordCount m a)
  deriving (Functor, Applicative, Monad)

-- | Run a 'LimitT', returning the value from the computation and the remaining
-- traversal limit.
runLimitT :: MonadThrow m => WordCount -> LimitT m a -> m (a, WordCount)
runLimitT limit (LimitT stateT) = runStateT stateT limit

-- | Run a 'LimitT', returning the value from the computation.
evalLimitT :: MonadThrow m => WordCount -> LimitT m a -> m a
evalLimitT limit (LimitT stateT) = evalStateT stateT limit

-- | Run a 'LimitT', returning the remaining traversal limit.
execLimitT :: MonadThrow m => WordCount -> LimitT m a -> m WordCount
execLimitT limit (LimitT stateT) = execStateT stateT limit

-- | A sensible default traversal limit. Currently 64 MiB.
defaultLimit :: WordCount
defaultLimit = (64 * 1024 * 1024) `div` 8

------ Instances of mtl type classes for 'LimitT'.

instance MonadThrow m => MonadThrow (LimitT m) where
  throwM = lift . throwM

instance MonadCatch m => MonadCatch (LimitT m) where
  catch (LimitT m) f = LimitT $ do
    catch m $ \e ->
      let LimitT m' = f e
       in m'

instance MonadThrow m => MonadLimit (LimitT m) where
  invoice deduct = LimitT $ do
    limit <- get
    when (limit < deduct) $ throwM TraversalLimitError
    put (limit - deduct)

instance MonadTrans LimitT where
  lift = LimitT . lift

instance MonadState s m => MonadState s (LimitT m) where
  get = lift get
  put = lift . put

instance (PrimMonad m, s ~ PrimState m) => PrimMonad (LimitT m) where
  type PrimState (LimitT m) = PrimState m
  primitive = lift . primitive

instance MonadFail m => MonadFail (LimitT m) where
  fail = lift . fail

instance MonadIO m => MonadIO (LimitT m) where
  liftIO = lift . liftIO

------ Instances of 'MonadLimit' for standard monad transformers

instance MonadLimit m => MonadLimit (StateT s m) where
  invoice = lift . invoice

instance MonadLimit m => MonadLimit (LazyState.StateT s m) where
  invoice = lift . invoice

instance (Monoid w, MonadLimit m) => MonadLimit (WriterT w m) where
  invoice = lift . invoice

instance (MonadLimit m) => MonadLimit (ReaderT r m) where
  invoice = lift . invoice

instance (Monoid w, MonadLimit m) => MonadLimit (RWST r w s m) where
  invoice = lift . invoice