From b2027f70d992bd2d088e71ee5cea7637445766f9 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <e.dolstra@tudelft.nl>
Date: Wed, 20 Jul 2011 18:10:47 +0000
Subject: * 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.
---
 src/libstore/derivations.cc | 79 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 79 insertions(+)

(limited to 'src/libstore/derivations.cc')

diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index e321ae8aaee6..db9fc6b8a561 100644
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@@ -2,11 +2,30 @@
 #include "store-api.hh"
 #include "globals.hh"
 #include "util.hh"
+#include "misc.hh"
 
 
 namespace nix {
 
 
+void DerivationOutput::parseHashInfo(bool & recursive, HashType & hashType, Hash & hash) const
+{
+    recursive = false;
+    string algo = hashAlgo;
+        
+    if (string(algo, 0, 2) == "r:") {
+        recursive = true;
+        algo = string(algo, 2);
+    }
+
+    hashType = parseHashType(algo);
+    if (hashType == htUnknown)
+        throw Error(format("unknown hash algorithm `%1%'") % algo);
+        
+    hash = parseHash(hashType, this->hash);
+}
+
+
 Path writeDerivation(const Derivation & drv, const string & name)
 {
     PathSet references;
@@ -171,4 +190,64 @@ bool isDerivation(const string & fileName)
 }
 
  
+bool isFixedOutputDrv(const Derivation & drv)
+{
+    return drv.outputs.size() == 1 &&
+        drv.outputs.begin()->first == "out" &&
+        drv.outputs.begin()->second.hash != "";
+}
+
+
+DrvHashes drvHashes;
+
+
+/* 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. */
+Hash hashDerivationModulo(Derivation drv)
+{
+    /* Return a fixed hash for fixed-output derivations. */
+    if (isFixedOutputDrv(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 = drvHashes[i->first];
+        if (h.type == htUnknown) {
+            Derivation drv2 = derivationFromPath(i->first);
+            h = hashDerivationModulo(drv2);
+            drvHashes[i->first] = h;
+        }
+        inputs2[printHash(h)] = i->second;
+    }
+    drv.inputDrvs = inputs2;
+    
+    return hashString(htSHA256, unparseDerivation(drv));
+}
+
+
 }
-- 
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