* 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, 21 insertions, 3 deletions

diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 4b04f5751ce8..d67ff2c77299 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -72,6 +72,13 @@ Path followLinksToStorePath(const Path & path)
 }
 
 
+string storePathToName(const Path & path)
+{
+    assertStorePath(path);
+    return string(path, nixStore.size() + 34);
+}
+
+
 void checkStoreName(const string & name)
 {
     string validChars = "+-._?=";
@@ -101,7 +108,9 @@ void checkStoreName(const string & name)
    
    <name> = a human readable name for the path, typically obtained
      from the name attribute of the derivation, or the name of the
-     source file from which the store path is created
+     source file from which the store path is created.  For derivation
+     outputs other than the default "out" output, the string "-<id>"
+     is suffixed to <name>.
      
    <h> = base-32 representation of the first 160 bits of a SHA-256
      hash of <s>; the hash part of the store name
@@ -120,11 +129,12 @@ void checkStoreName(const string & name)
      "source"
        for paths copied to the store using addToStore() when recursive
        = true and hashAlgo = "sha256"
-     "output:out"
+     "output:<id>"
        for either the outputs created by derivations, OR paths copied
        to the store using addToStore() with recursive != true or
        hashAlgo != "sha256" (in that case "source" is used; it's
-       silly, but it's done that way for compatibility).
+       silly, but it's done that way for compatibility).  <id> is the
+       name of the output (usually, "out").
 
    <h2> = base-16 representation of a SHA-256 hash of:
      if <type> = "text:...":
@@ -174,6 +184,14 @@ Path makeStorePath(const string & type,
 }
 
 
+Path makeOutputPath(const string & id,
+    const Hash & hash, const string & name)
+{
+    return makeStorePath("output:" + id, hash,
+        name + (id == "out" ? "" : "-" + id));
+}
+
+
 Path makeFixedOutputPath(bool recursive,
     HashType hashAlgo, Hash hash, string name)
 {