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Safely running untrusted Haskell code

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(= The implementation)

Revision as of 05:45, 27 May 2007

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_channel|IRC channels], in particular,

  1. haskell. 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

The evaluator is essentially a function,
eval :: String -> IO
, 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:

  • parse the expression to check it is a Haskell 98 expression

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 stToIO and such like.
  • Module imports are disallowed
  • Time and space limitations on the runplugs process are set by the OS
  • The expression is bound to a random top level identifier (harmless to guess)
  • The expression is wrapped in 'show', and must be an instance of Show
  • An instance of Show IO is defined, which prints "<IO>", rendering IO impossible.
  • The expression is type checked, with the show constraint, enforcing purity
  • If it type checks, and the type checker doesn't time out, it is compiled to native code with -fasm
  • Only -fextended-default-rules are allowed, as language extensions over H98.
  • The resulting .o file is dynamically linked into the throw-away runplugs instance
  • The value is evaluated inside an exception handler.
  • If an exception is thrown, only the first 1024 bytes of the exception string are returned.
  • If all went well, the first 2048 bytes of the shown string are returned to the caller.

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 typechecker not terminate
  • using pathological type inference cases to have the type checker not terminate
  • code injection of code fragments that arne'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
  • 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.

3 Template Haskell

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

4 See also