LIO.Core

This module implements the core of the Labeled IO (LIO) library for information flow control (IFC) in Haskell. It provides a monad, LIO, that is intended to be used as a replacement for the IO monad in untrusted code. The idea is for untrusted code to provide a computation in the LIO monad, which trusted code can then safely execute through using evalLIO-like functions. Though, usually a wrapper function is employed depending on the type of labels used by an application. For example, with LIO.DCLabel trusted code would evalDC to execute an untrusted computation.

Labels are a way of describing who can observe and modify data. (A detailed consideration of labels is given in LIO.Label.) LIO associates a current label with every LIO computation. The current label effectively tracks the sensitivity of all the data that the computation has observed. For example, if the computation has read a "secret" mutable refernce (see LIO.LIORef) and then the result of a "top-secret" thread (see LIO.Concurrent) then the current label will be at least "top-secret". The role of the current label is two-fold. First, the current label protects all the data in scope -- it is the label associated with any unlabeled data. For example, the current label is the label on constants such as 3 or "tis a string". More interestingly, consider reading a "secret" file:

 bs <- readFile "/secret/file.txt"

Though the label in the file store may be "secret", bs has type ByteString, which is not explicitly labeled. Hence, to protect the contents (bs) the current label must be at least "secret" before executing readFile. More generally, if the current label is L_cur, then it is only permissible to read data labeled L_r if L_r `canFlowTo` L_cur. Note that, rather than throw an exception, reading data will often just increase the current label to ensure that L_r `canFlowTo` L_cur using taint.

Second, the current label prevents inforation leaks into public channels. Specifically, it is only permissible to modify, or write to, data labeled L_w when L_cur`canFlowTo` L_w. Thus, it the following attempt to leak the secret bs would fail:

 writeFile "/public/file.txt" bs

In addition to the current label, the LIO monad keeps a second label, the current clearance (accessible via the getClearance function). The clearance can be used to enforce a "need-to-know" policy since it represents the highest value the current label can be raised to. In other words, if the current clearance is L_clear then the computation cannot create, read or write to objects labeled L such that L `canFlowTo` L_clear does not hold.

This module exports the LIO monad, functions to access the internal state (e.g., getLabel and getClearance), functions for raising and catching exceptions, and IFC guards. Exceptions are core to LIO since they provide a way to deal with potentially-misbehaving untrusted code. Specifically, when a computation is about to violate IFC (as writeFile above), an exception is raised. Guards provide a useful abstraction for dealing with labeled objects; they should be used before performing a read-only, write-only, or read-write operation on a labeled object. The remaining core, but not all, abstractions are exported by LIO.

LIO monad

data LIO l a

The LIO monad is a state monad, with IO as the underlying monad, that carries along a current label (lioLabel) and current clearance (lioClearance). The current label imposes restrictions on what the current computation may read and write (e.g., no writes to public channels after reading sensitive data). Since the current label can be raised to be permissive in what a computation observes, we need a way to prevent certain computations from reading overly sensitive data. This is the role of the current clearance: it imposes an upper bound on the current label.

Instances

Typeable2 LIO

Label l => MonadLIO l (LIO l)

GuardIO l (IO r) (LIO l r)

GuardIO l (a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> a7 -> a8 -> a9 -> a10 -> IO r) (a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> a7 -> a8 -> a9 -> a10 -> LIO l r)

GuardIO l (a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> a7 -> a8 -> a9 -> IO r) (a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> a7 -> a8 -> a9 -> LIO l r)

GuardIO l (a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> a7 -> a8 -> IO r) (a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> a7 -> a8 -> LIO l r)

GuardIO l (a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> a7 -> IO r) (a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> a7 -> LIO l r)

GuardIO l (a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> IO r) (a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> LIO l r)

GuardIO l (a1 -> a2 -> a3 -> a4 -> a5 -> IO r) (a1 -> a2 -> a3 -> a4 -> a5 -> LIO l r)

GuardIO l (a1 -> a2 -> a3 -> a4 -> IO r) (a1 -> a2 -> a3 -> a4 -> LIO l r)

GuardIO l (a1 -> a2 -> a3 -> IO r) (a1 -> a2 -> a3 -> LIO l r)

GuardIO l (a1 -> a2 -> IO r) (a1 -> a2 -> LIO l r)

GuardIO l (a1 -> IO r) (a1 -> LIO l r)

Monad (LIO l)

Functor (LIO l)

Applicative (LIO l)

class (Monad m, Label l) => MonadLIO l m | m -> l where

Synonym for monad in which LIO is the base monad.

Methods

liftLIO :: LIO l a -> m a

Lift an LIO computation.

Instances

Label l => MonadLIO l (LIO l)

Execute LIO actions

data LIOState l

Internal state of an LIO computation.

Constructors

LIOState

Fields

lioLabel :: !l

Current label.

lioClearance :: !l

Current clearance.

Instances

Eq l => Eq (LIOState l)

Read l => Read (LIOState l)

Show l => Show (LIOState l)

evalLIO :: LIO l a -> LIOState l -> IO a

Given an LIO computation and some initial state, return an IO action which when executed will perform the IFC-safe LIO computation.

Because untrusted code cannot execute IO computations, this function should only be useful within trusted code. No harm is done from exposing the evalLIO symbol to untrusted code. (In general, untrusted code is free to produce IO computations, but it cannot execute them.)

Unlike runLIO, this function throws an exception if the underlying LIO action terminates with an exception.

runLIO :: LIO l a -> LIOState l -> IO (a, LIOState l)

Execute an LIO action, returning its result and the final label state as a pair. Note that it returns a pair whether or not the LIO action throws an exception. Forcing the result value will re-throw the exception, but the label state will always be valid.

See also evalLIO.

Manipulating label state

getLabel :: Label l => LIO l l

Returns the current value of the thread's label.

setLabel :: Label l => l -> LIO l ()

Raise the current label to the provided label, which must be between the current label and clearance. See taint.

setLabelP :: PrivDesc l p => Priv p -> l -> LIO l ()

If the current label is oldLabel and the current clearance is clearance, this function allows code to raise the current label to any value newLabel such that oldLabel `canFlowTo` newLabel && newLabel `canFlowTo` clearance.

Manipulating clearance

getClearance :: Label l => LIO l l

Returns the current value of the thread's clearance.

setClearance :: Label l => l -> LIO l ()

Lower the current clearance. The new clerance must be between the current label and clerance. One cannot raise the current label or create object with labels higher than the current clearance.

setClearanceP :: PrivDesc l p => Priv p -> l -> LIO l ()

Raise the current clearance (undoing the effects of setClearance) by exercising privileges. If the current label is l and current clearance is c, then setClearanceP p cnew succeeds only if the new clearance is can flow to the current clearance (modulo privileges), i.e., canFlowToP p cnew c must hold. Additionally, the current label must flow to the new clearance, i.e., l `canFlowTo` cnew must hold.

scopeClearance :: Label l => LIO l a -> LIO l a

Runs an LIO action and re-sets the current clearance to its previous value once the action returns. In particular, if the action lowers the current clearance, the clearance will be restored upon return.

Note that scopeClearance always restores the clearance. If that causes the clearance to drop below the current label, a ClearanceViolation exception is thrown. That exception can only be caught outside a second scopeClearance that restores the clearance to higher than the current label.

withClearance :: Label l => l -> LIO l a -> LIO l a

Lowers the clearance of a computation, then restores the clearance to its previous value (actually, to the upper bound of the current label and previous value). Useful to wrap around a computation if you want to be sure you can catch exceptions thrown by it. The supplied clearance label must be bounded by the current label and clearance as enforced by guardAlloc.

Note that if the computation inside withClearance acquires any Privs, it may still be able to raise its clearance above the supplied argument using setClearanceP.

withClearanceP :: PrivDesc l p => Priv p -> l -> LIO l a -> LIO l a

Same as withClearance, but uses privileges when applying guardAllocP to the supplied label.

Exceptions thrown by LIO

Library functions throw an exceptions before an IFC violation can take place. MonitorFailure should be used when the reason for failure is sufficiently described by the type. Otherwise, VMonitorFailure (i.e., "Verbose"-MonitorFailure) should be used to further describe the error.

data MonitorFailure

Exceptions thrown when some IFC restriction is about to be violated.

Constructors

ClearanceViolation

Current label would exceed clearance, or object label is above clearance.

CurrentLabelViolation

Clearance would be below current label, or object label is not above current label.

InsufficientPrivs

Insufficient privileges. Thrown when lowering the current label or raising the clearance cannot be accomplished with the supplied privileges.

CanFlowToViolation

Generic can-flow-to failure, use with VMonitorFailure

ResultExceedsLabel

Instances

Show MonitorFailure

Typeable MonitorFailure

Exception MonitorFailure

data VMonitorFailure

Verbose version of MonitorFailure also carrying around a detailed message.

Constructors

VMonitorFailure

Fields

monitorFailure :: MonitorFailure

Generic monitor failure.

monitorMessage :: String

Detailed message of failure.

Instances

Show VMonitorFailure

Typeable VMonitorFailure

Exception VMonitorFailure

Guards

Guards are used by (usually privileged) code to check that the invoking, unprivileged code has access to particular data. If the current label is lcurrent and the current clearance is ccurrent, then the following checks should be performed when accessing data labeled ldata:

• When reading an object labeled ldata, it must be the case that ldata `canFlowTo` lcurrent. This check is performed by the taint function, so named because it "taints" the current LIO context by raising lcurrent until ldata `canFlowTo` lcurrent. (Specifically, it does this by computing the least upperBound of the two labels.) However, this is done only if the new lcurrent `canFlowTo` ccurrent.
• When creating or allocating objects, it is permissible for them to be higher than the current label, so long as they are below the current clearance. In other words, it must be the case that lcurrent `canFlowTo` ldata && ldata `canFlowTo` ccurrent. This is ensured by the guardAlloc function.
• When writing an object, it should be the case that lcurrent `canFlowTo` ldata && ldata `canFlowTo` lcurrent. (As stated, this is the same as saying ldata == lcurrent, but the two are different when using canFlowToP instead of canFlowTo.) This is ensured by the guardWrite function, which does the equivalent of taint to ensure the target label ldata can flow to the current label, then throws an exception if lcurrent cannot flow back to the target label.

Note that in this case a write always implies a read. Hence, when writing to an object for which you can observe the result, you must use guardWrite. However, when performing a write for which there is no observable side-effects to the writer, i.e., you cannot observe the success or failure of the write, then it is safe to solely use guardAlloc.

The taintP, guardAllocP, and guardWriteP functions are variants of the above that take privilege to be more permissive and raise the current label less.

guardAlloc :: Label l => l -> LIO l ()

Ensures the label argument is between the current IO label and current IO clearance. Use this function in code that allocates objects--untrusted code shouldn't be able to create an object labeled l unless guardAlloc l does not throw an exception. Similarly use this guard in any code that writes to an object labeled l for which the write has no observable side-effects.

If the label does not flow to clearance ClearanceViolation is thrown; if the current label does not flow to the argument label CurrentLabelViolation is thrown.

guardAllocP :: PrivDesc l p => Priv p -> l -> LIO l ()

Like guardAlloc, but takes privilege argument to be more permissive. Note: privileges are only used when checking that the current label can flow to the given label.

Read-only

taint :: Label l => l -> LIO l ()

Use taint l in trusted code before observing an object labeled l. This will raise the current label to a value l' such that l `canFlowTo` l', or throw ClearanceViolation if l' would have to be higher than the current clearance.

taintP :: PrivDesc l p => Priv p -> l -> LIO l ()

Like taint, but use privileges to reduce the amount of taint required. Note that taintP will never lower the current label. It simply uses privileges to avoid raising the label as high as taint would raise it.

Write

guardWrite :: Label l => l -> LIO l ()

Use guardWrite l in any (trusted) code before modifying an object labeled l, for which a the modification can be observed, i.e., the write implies a read.

The implementation of guardWrite is straight forward:

 guardWrite l = taint l >> guardAlloc l

This guarantees that l `canFlowTo` the current label (and clearance), and that the current label `canFlowTo` l.

guardWriteP :: PrivDesc l p => Priv p -> l -> LIO l ()

Like guardWrite, but takes privilege argument to be more permissive.