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Safely running untrusted Haskell code

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Obviously, don't run code in the IO monad, just show pure results (or possibly make your own monad that is a restricted subset of IO). But it's a lot more complicated than that...


1 Verifying safety : lambdabot's approach

Since 2004, lambdabot has executed arbitrary strings of Haskell provided by user's of various IRC channels, in particular, the Haskell channel. In order to do this, a particular security policy is required. The policy, and its implementation, is described here.

1.1 The policy

Only allow execution of pure Haskell expressions.

1.2 The implementation

Note: This section refers to the old Lambdabot evaluator; as of 2009, lambdabot calls out to mueval, which while it uses many of the same techniques, is structured differently.
The evaluator is essentially a function,
eval :: String -> IO String
, which takes a random Haskell string, verifies it,

compiles it, and evaluates the result, returning a String representing the result, back over the network.

This function is implemented as two separate processes:

The driver reads a String from the network, and then subjects it to a simple test:

  • The expression is parsed as a Haskell 98 expression, hopefully preventing code injection (is this true? and can any string that can parse as a valid Haskell expression become something more sinister when put in a particular context?)

If the string parses as a Haskell 98 expression, the 'runplugs' process is then forked to evaluate the string, and the following checks are put in place:

  • Only a trusted module set is imported, avoiding unsafePerformIO and unsafeIOtoST and such like.
  • Module imports are disallowed
  • Time and space limitations on the runplugs process are set by the OS 'rlimit' facility
  • The expression type checked, enforcing lack of memory errors
  • Because the user code is not at the beginning of the file, malicious {-# LANGUAGE #-} and {-# OPTIONS #-} flags are ignored
  • Only -fextended-default-rules are allowed, as language extensions over H98.
  • The resulting .o file is dynamically linked only into the throw-away runplugs instance
  • Even if all went well, the first 2048 characters of the shown string are returned to the caller (no infinite output DoS)

A few other niceties are provided:

  • The expression is bound to a random identifier (harmless to guess), in order to allow nice line error messages with line pragmas.
  • The expression is wrapped in 'show'.
  • A catch-all instance of Show in terms of Typable is provided, to display non-displayable objects in a more useful way (e.g. putStrLn --> <[Char] -> IO ()>)
  • It is compiled to native code with -fasm for speed (compilation time is neglible compared to IRC lag)
  • The value is evaluated inside an exception handler; if an exception is thrown, the first 1024 characters of the exception string are returned.

2 Exploits

A variety of interesting exploits have been found, or thought of, over the years. Those we remember are listed below:

  • using newtype recursion to have the inliner not terminate
  • using pathological type inference cases to have the type checker not terminate
  • code injection of code fragments that aren't Haskell expressions
  • Template Haskell used to run IO actions during type checking
  • stToIO to convert a safe ST action, into an IO action that is run
  • large strings returned in exceptions
  • unsafePerformIO, of course
  • unsafeCoerce#
  • throwing a piece of code as an exception, which is evaluated when the exception is shown
  • non-terminating code, in a tight loop that doesn't allocate, can't use GHC's threadDelay/scheduler (let f () = f () in f ()) to timeout (must use OS resource limits).
  • large array allocations can fill memory
  • very large array allocations can integer overflow the storage manager, allowing arbitrary memory access (this appears to be fixed in GHC 6.8.x)
  • creating class instances that violate assumed laws (cf EvilIx)
  • various literal strings that print IRC protocol commands could be printed using exceptions.
  • if a user guesses the top level identifier the expression is bound to, it can be used to print a silly string
  • zombies could be created by multiple runplugs calls, leading to blocking on endless output. the resulting zombies accumulate, eventually leading to DOS. (if waitForProcess was broken)

3 Template Haskell

We believe that Template Haskell can be made safe for users by hiding runIO and reify.

4 See also