Version 10 (modified by David, 6 years ago) (diff)


Safe Haskell

This is a proposal for a Haskell extension through which people can safely execute untrusted Haskell code, much the way web browsers currently run untrusted Java and JavaScript, or the way the Spin and Singularity operating systems ran untrusted Modula-3 and C#/Sing#.


Safe Haskell assumes the following setup.

  • A server S wants to run code provided by untrusted (and perhaps malicious) clients X.
  • Clients X may send untrusted Haskell code to S in source form
  • The server compiles this untrusted code, with the -XSafe flag.
  • If compilation succeeds, S can safely run the code, knowing that it cannot cause unsafe effects.

More specifically, there are two parts to this proposed extension:

  1. An option to GHC (-XSafe) that causes it to reject any source code that might produce unsafe effects.
  1. An option to GHC (-XTrusted) indicating that, even though a module might invoke unsafe functions internally, the set of exported symbols cannot be used in an unsafe way.

A module compiled with -XSafe can only import modules compiled with -XTrusted or -XSafe.

Safety Goal

As long as no module compiled with -XTrusted contains a vulnerability, the goal of -XSafe is to guarantee the following properties:

  • Referential transparency. Functions in a module compiled -XSafe must be deterministic. Moreover, evaluating them should have no side effects, and should not halt the program (except by throwing uncaught exceptions or looping forever).
  • Constructor access control. Code in a module compiled -XSafe must not be able to examine or synthesize data values using constructors the module cannot import.

Note that -XSafe should not prevent use of the symbol IO. Authors of normal (trusted) code may wish to use {-# LANGUAGE Safe #-} as a means of ensuring they do not accidentally invoke unsafe actions, directly or indirectly.

Of course, if an untrusted module exports an IO action, that action may have arbitrary side effects. Compiling the module with -XSafe does not meaningfully restrict the effects of exported IO actions. Hence, an application importing an untrusted but safe module may safely invoke pure functions from the untrusted module, but must avoid executing IO actions from the module.


The following aspects of Haskell can be used to violate the safety goal, and thus would need to be disallowed when -XSafe is in effect. Please add more examples to this list, as some are likely missing.

  • Some symbols in GHC.Prim can be used to do very unsafe things. At least one of these symbols, realWorld#, is magically exported by GHC.Prim even though it doesn't appear in the GHC.Prim module export list. Are there other such magic symbols in this or other modules?
  • A number of functions can be used to violate safety. Many of these have names prefixed with unsafe (e.g., unsafePerformIO, unsafeIterleaveIO, unsafeCoerce unsafeSTToIO, ...). However, not all unsafe functions fit this pattern. For instance, inlinePerformIO and fromForeignPtr from the bytestring package are unsafe.
  • Code that defines hand-crafted instances of Typeable can violate safety by causing typeOf to return identical results on two distinct types, then using cast to coerce between the two unsafely. Are there other classes? Perhaps Data should also be restricted? Simon says Ix doesn't need to be protected any more.
  • Certain exposed constructors of otherwise mostly safe data types allow unsafe actions. For instance, the PS constructor of Data.ByteString.ByteString contains a pointer, offset, and length. Code that can see the pointer value can act in a non-deterministic way by depending on the address rather than value of a ByteString. Worse, code that can use PS to construct ByteStrings can include bad lengths that will lead to stray pointer references.
  • The FFI extension is inherently unsafe.
  • TemplateHaskell is also particularly dangerous, as it can cause side effects even at compilation time.
  • The OverlappingInstances extension may allow unsafe actions, because untrusted code can potentially redefine a type instance (by containing a more specific instance definition) in a way that changes the behavior of code importing the untrusted module. SLPJ: this may be undesirable, but does it violate the Safety Goal?
  • Likewise, RULES and SPECIALIZE pragmas can change the behavior of trusted code in unanticipated ways. SLPJ: same question
  • OPTIONS_GHC is dangerous in unfiltered form, as it could potentially expose packages with trusted but not trustworthy modules. -XSafe must be processed last after all other options. If previous options conflict with -XSafe, they must be overrided or compilation must fail.
  • The StandaloneDeriving extension can be used to violate constructor access control by defining instances of Read and Show to examine and construct data values with inaccessible constructors.
  • Similarly, GeneralizedNewtypeDeriving can also violate constructor access control, by allowing untrusted code to manipulate protected data types in ways the data type author did not intend.

Implementation details

  • An interface file should record whether a module is safe. When the module is safe, the interface file should additionally include a set of trusted modules on which the module depends. SLPJ:what is the function of the "set of trusted modules on which it depends"? There could be some option like --trust-only that restricts the set of packages from which trusted modules may be imported. Thus one could restrict what modules safe code imports in a way that is independent of whatever happens to be installed in a user's ~/.cabal directory.
  • A module compiled with -XTrusted should be marked safe; its set of trusted modules should contain itself and only itself.
  • A module compiled with -XSafe should only be able to import modules that are marked safe. Its set of trusted modules should be the union of the trusted sets of all the modules it imports.
  • GHC.Prim will need to be made (or just kept) unsafe.
  • -XSafe should disallow the FFI, TemplateHaskell, OverlappingInstances, StandaloneDeriving, GeneralizedNewtypeDeriving, and CPP language extensions, as well as RULES and SPECIALIZE pragmas.
  • OPTIONS_GHC pragmas will have to be filtered. Some options, (e.g., -fno-warn-unused-do-bind) are totally fine, but many others are likely problematic (e.g., -cpp, which provides access to the local file system at compilation time, or -F which allows an arbitrary file to be executed, possibly even one named /afs/... and hence entirely under an attacker's control).
  • Libraries will progressively need to be updated to export safe interfaces, which may require moving unsafe functions into separate modules, or adding new {-# LANGUAGE Safe #-} modules that re-export a safe subset of symbols. Ideally, most modules in widely-used libraries would eventually contain either {-# LANGUAGE Safe -#} or {-# LANGUAGE Trusted -#} pragmas, except for internal modules or a few modules exporting unsafe symbols. Maybe haddock could add some indicator to make it obvious which modules are safe.
  • The -XTrusted command-line option and corresponding pragma do not increase what a module can do--the only effect is to mark the module's interface file as safe. In particular, if both -XTrusted and -XSafe are supplied, then any unsafe actions will still cause a compilation error. A plausible use for both pragmas simultaneously is to prune the list of trusted modules--for instance if a module imports a bunch of trusted modules but does not use any of their trusted features, or only uses those features in a very limited way. If the code happens also to be safe, the programmer may want to add -XSafe to catch accidental unsafe actions.
  • It might be nice to add a third option, {-# LANGUAGE Unsafe -#} , which prevents a module from compiling with -XSafe and disables the -XTrusted option so that the interface file is guaranteed not to be marked safe. This option could be used in seemingly safe modules that export constructors that would cause other modules to do unsafe things. (The PS constructor discussed above is an example of a dangerous constructor that could potentially be defined in a module that happily compiles with -XSafe.) This isn't strictly necessary, since one could always just enable another extension such as FFI, but having a specific Unsafe language option seems cleaner.


The following links are to discussions of similar topics: