* Fix a huuuuge security hole in the Nix daemon. It didn't check that

  derivations added to the store by clients have "correct" output
  paths (meaning that the output paths are computed by hashing the
  derivation according to a certain algorithm).  This means that a
  malicious user could craft a special .drv file to build *any*
  desired path in the store with any desired contents (so long as the
  path doesn't already exist).  Then the attacker just needs to wait
  for a victim to come along and install the compromised path.

  For instance, if Alice (the attacker) knows that the latest Firefox
  derivation in Nixpkgs produces the path

    /nix/store/1a5nyfd4ajxbyy97r1fslhgrv70gj8a7-firefox-5.0.1

  then (provided this path doesn't already exist) she can craft a .drv
  file that creates that path (i.e., has it as one of its outputs),
  add it to the store using "nix-store --add", and build it with
  "nix-store -r".  So the fake .drv could write a Trojan to the
  Firefox path.  Then, if user Bob (the victim) comes along and does

    $ nix-env -i firefox
    $ firefox

  he executes the Trojan injected by Alice.

  The fix is to have the Nix daemon verify that derivation outputs are
  correct (in addValidPath()).  This required some refactoring to move
  the hash computation code to libstore.

1 files changed, 3 insertions, 61 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index e58f9265f0..c01bd6e54f 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -246,63 +246,6 @@ static void prim_trace(EvalState & state, Value * * args, Value & v)
  *************************************************************/
 
 
-static bool isFixedOutput(const Derivation & drv)
-{
-    return drv.outputs.size() == 1 &&
-        drv.outputs.begin()->first == "out" &&
-        drv.outputs.begin()->second.hash != "";
-}
-
-
-/* Returns the hash of a derivation modulo fixed-output
-   subderivations.  A fixed-output derivation is a derivation with one
-   output (`out') for which an expected hash and hash algorithm are
-   specified (using the `outputHash' and `outputHashAlgo'
-   attributes).  We don't want changes to such derivations to
-   propagate upwards through the dependency graph, changing output
-   paths everywhere.
-
-   For instance, if we change the url in a call to the `fetchurl'
-   function, we do not want to rebuild everything depending on it
-   (after all, (the hash of) the file being downloaded is unchanged).
-   So the *output paths* should not change.  On the other hand, the
-   *derivation paths* should change to reflect the new dependency
-   graph.
-
-   That's what this function does: it returns a hash which is just the
-   hash of the derivation ATerm, except that any input derivation
-   paths have been replaced by the result of a recursive call to this
-   function, and that for fixed-output derivations we return a hash of
-   its output path. */
-static Hash hashDerivationModulo(EvalState & state, Derivation drv)
-{
-    /* Return a fixed hash for fixed-output derivations. */
-    if (isFixedOutput(drv)) {
-        DerivationOutputs::const_iterator i = drv.outputs.begin();
-        return hashString(htSHA256, "fixed:out:"
-            + i->second.hashAlgo + ":"
-            + i->second.hash + ":"
-            + i->second.path);
-    }
-
-    /* For other derivations, replace the inputs paths with recursive
-       calls to this function.*/
-    DerivationInputs inputs2;
-    foreach (DerivationInputs::const_iterator, i, drv.inputDrvs) {
-        Hash h = state.drvHashes[i->first];
-        if (h.type == htUnknown) {
-            Derivation drv2 = derivationFromPath(i->first);
-            h = hashDerivationModulo(state, drv2);
-            state.drvHashes[i->first] = h;
-        }
-        inputs2[printHash(h)] = i->second;
-    }
-    drv.inputDrvs = inputs2;
-    
-    return hashString(htSHA256, unparseDerivation(drv));
-}
-
-
 /* Construct (as a unobservable side effect) a Nix derivation
    expression that performs the derivation described by the argument
    set.  Returns the original set extended with the following
@@ -497,11 +440,10 @@ static void prim_derivationStrict(EvalState & state, Value * * args, Value & v)
        (‘i->first == "out" ...’) doesn't affect the hash of the
        others.  Is that exploitable? */
     if (!fixedOnly) {
-        Hash h = hashDerivationModulo(state, drv);
+        Hash h = hashDerivationModulo(drv);
         foreach (DerivationOutputs::iterator, i, drv.outputs)
             if (i->second.path == "") {
-                Path outPath = makeStorePath("output:" + i->first, h,
-                    drvName + (i->first == "out" ? "" : "-" + i->first));
+                Path outPath = makeOutputPath(i->first, h, drvName);
                 drv.env[i->first] = outPath;
                 i->second.path = outPath;
             }
@@ -516,7 +458,7 @@ static void prim_derivationStrict(EvalState & state, Value * * args, Value & v)
     /* Optimisation, but required in read-only mode! because in that
        case we don't actually write store derivations, so we can't
        read them later. */
-    state.drvHashes[drvPath] = hashDerivationModulo(state, drv);
+    drvHashes[drvPath] = hashDerivationModulo(drv);
 
     state.mkAttrs(v, 1 + drv.outputs.size());
     mkString(*state.allocAttr(v, state.sDrvPath), drvPath, singleton<PathSet>("=" + drvPath));