Safe Haskell is designed to give users enough guarantees about the safety properties of compiled code so that secure systems can be built using Haskell. A lot of work has been done with Haskell, building such systems as information flow control security, capability based security, DSLs for working with encrypted data... etc. These systems all rely on properties of the Haskell language that aren't true in the general case where uses of functions like unsafePerformIO are allowed.

As an example lets define an interface for a plugin system where the plugin authors are untrusted, possibly malicious third-parties. We do this by restricting the plugin interface to pure functions or to a restricted IO monad that we have defined that only allows a safe subset of IO actions to be executed. We define the plugin interface here so that it requires the plugin module, Danger, to export a single computation, Danger.runMe, of type RIO (), where RIO is a new monad defined as follows:

        -- Either of the following Safe Haskell pragmas would do
        {-# LANGUAGE Trustworthy #-}
        {-# LANGUAGE Safe #-}

        module RIO (RIO(), runRIO, rioReadFile, rioWriteFile) where

        -- Notice that symbol UnsafeRIO is not exported from this module!
        newtype RIO a = UnsafeRIO { runRIO :: IO a }

        instance Monad RIO where
            return = UnsafeRIO . return
            (UnsafeRIO m) >>= k = UnsafeRIO $ m >>= runRIO . k

        -- Returns True iff access is allowed to file name
        pathOK :: FilePath -> IO Bool
        pathOK file = {- Implement some policy based on file name -}

        rioReadFile :: FilePath -> RIO String
        rioReadFile file = UnsafeRIO $ do
          ok <- pathOK file
          if ok then readFile file else return ""

        rioWriteFile :: FilePath -> String -> RIO ()
        rioWriteFile file contents = UnsafeRIO $ do
          ok <- pathOK file
          if ok then writeFile file contents else return ()
      

        {-# LANGUAGE Safe #-}
        module Danger ( runMe ) where

        runMe :: RIO ()
        runMe = ...
      

To stop these attacks Safe Haskell can be used. This is done by compiling the RIO module with the -XTrustworthy flag and compiling the Danger module with the -XSafe flag.

The use of the -XSafe flag to compile the Danger module restricts the features of Haskell that can be used to a safe subset. This includes disallowing unsafePerfromIO, Template Haskell, pure FFI functions, Generalized Newtype Deriving, RULES and restricting the operation of Overlapping Instances. The -XSafe flag also restricts the modules can be imported by Danger to only those that are considered trusted. Trusted modules are those compiled with -XSafe, where GHC provides a mechanical guarantee that the code is safe. Or those modules compiled with -XTrustworthy, where the module author claims that the module is Safe.

This is why the RIO module is compiled with -XTrustworthy, to allow the Danger module to import it. The -XTrustworthy flag doesn't place any restrictions on the module like -XSafe does. Instead the module author claims that while code may use unsafe features internally, it only exposes an API that can used in a safe manner. There is an issue here as -XTrustworthy may be used by an arbitrary module and module author. Because of this for trustworthy modules to be considered trusted, and so allowed to be used in -XSafe compiled code, the client C compiling the code must tell GHC that they trust the package the trustworthy module resides in. This is essentially a way of for C to say, while this package contains trustworthy modules that can be used by untrusted modules compiled with -XSafe , I trust the author(s) of this package and trust the modules only expose a safe API. The trust of a package can be changed at any time, so if a vulnerability found in a package, C can declare that package untrusted so that any future compilation against that package would fail. For a more detailed overview of this mechanism see Section 7.20.4, “Trust”.

So Danger can import module RIO because RIO is marked trustworthy. Thus, Danger can make use of the rioReadFile and rioWriteFile functions to access permitted file names. The main application then imports both RIO and Danger. To run the plugin, it calls RIO.runRIO Danger.runMe within the IO monad. The application is safe in the knowledge that the only IO to ensue will be to files whose paths were approved by the pathOK test.

        Package Wuggle:
           {-# LANGUAGE Safe #-}
           module Buggle where
             import Prelude
             f x = ...blah...

        Package P:
           {-# LANGUAGE Trustworthy #-}
           module M where
             import System.IO.Unsafe
             import safe Buggle
      

Suppose a client C decides to trust package P. Then does C trust module M? To decide, GHC must check M's imports — M imports System.IO.Unsafe. M was compiled with -XTrustworthy, so P's author takes responsibility for that import. C trusts P's author, so C trusts M to only use its unsafe imports in a safe and consistent manner with respect to the API M exposes. M also has a safe import of Buggle, so for this import P's author takes no responsibility for the safety, so GHC must check whether Buggle is trusted by C. Is it? Well, it is compiled with -XSafe, so the code in Buggle itself is machine-checked to be OK, but again under the assumption that all of Buggle's imports are trusted by C. Prelude comes from base, which C trusts, and is compiled with -XTrustworthy (While Prelude is typically imported implicitly, it still obeys the same rules outlined here). So Buggle is considered trusted.

Notice that C didn't need to trust package Wuggle; the machine checking is enough. C only needs to trust packages that contain -XTrustworthy modules.

        import safe Untrusted.Module