Add restricted evaluation mode

If ‘--option restrict-eval true’ is given, the evaluator will throw an
exception if an attempt is made to access any file outside of the Nix
search path. This is primarily intended for Hydra, where we don't want
people doing ‘builtins.readFile ~/.ssh/id_dsa’ or stuff like that.

doc/manual/command-ref/conf-file.xml

@@ -539,6 +539,21 @@ flag, e.g. <literal>--option gc-keep-outputs false</literal>.</para>
   </varlistentry>
 
 
+  <varlistentry xml:id="conf-restrict-eval"><term><literal>restrict-eval</literal></term>
+
+    <listitem>
+
+      <para>If set to <literal>true</literal>, the Nix evaluator will
+      not allow access to any files outside of the Nix search path (as
+      set via the <envar>NIX_PATH</envar> environment variable or the
+      <option>-I</option> option). The default is
+      <literal>false</literal>.</para>
+
+    </listitem>
+
+  </varlistentry>
+
+
 </variablelist>
 
 </para>

src/libexpr/eval.cc

@@ -195,6 +195,8 @@ EvalState::EvalState(const Strings & _searchPath)
     nrListConcats = nrPrimOpCalls = nrFunctionCalls = 0;
     countCalls = getEnv("NIX_COUNT_CALLS", "0") != "0";
 
+    restricted = settings.get("restrict-eval", false);
+
 #if HAVE_BOEHMGC
     static bool gcInitialised = false;
     if (!gcInitialised) {
@@ -250,6 +252,21 @@ EvalState::~EvalState()
 }
 
 
+Path EvalState::checkSourcePath(const Path & path_)
+{
+    if (!restricted) return path_;
+
+    /* Resolve symlinks. */
+    Path path = canonPath(path_, true);
+
+    for (auto & i : searchPath)
+        if (path == i.second || isInDir(path, i.second))
+            return path;
+
+    throw RestrictedPathError(format("access to path ‘%1%’ is forbidden in restricted mode") % path_);
+}
+
+
 void EvalState::addConstant(const string & name, Value & v)
 {
     Value * v2 = allocValue();
@@ -555,7 +572,7 @@ void EvalState::evalFile(const Path & path, Value & v)
     }
 
     startNest(nest, lvlTalkative, format("evaluating file ‘%1%’") % path2);
-    Expr * e = parseExprFromFile(path2);
+    Expr * e = parseExprFromFile(checkSourcePath(path2));
     try {
         eval(e, v);
     } catch (Error & e) {
@@ -1358,8 +1375,8 @@ string EvalState::copyPathToStore(PathSet & context, const Path & path)
         dstPath = srcToStore[path];
     else {
         dstPath = settings.readOnlyMode
-            ? computeStorePathForPath(path).first
+            ? computeStorePathForPath(checkSourcePath(path)).first
-            : store->addToStore(path, true, htSHA256, defaultPathFilter, repair);
+            : store->addToStore(checkSourcePath(path), true, htSHA256, defaultPathFilter, repair);
         srcToStore[path] = dstPath;
         printMsg(lvlChatty, format("copied source ‘%1%’ -> ‘%2%’")
             % path % dstPath);

src/libexpr/eval.hh

@@ -135,6 +135,10 @@ public:
        already exist there. */
     bool repair;
 
+    /* If set, don't allow access to files outside of the Nix search
+       path or to environment variables. */
+    bool restricted;
+
 private:
     SrcToStore srcToStore;
 
@@ -155,6 +159,8 @@ public:
 
     void addToSearchPath(const string & s, bool warn = false);
 
+    Path checkSourcePath(const Path & path);
+
     /* Parse a Nix expression from the specified file. */
     Expr * parseExprFromFile(const Path & path);
     Expr * parseExprFromFile(const Path & path, StaticEnv & staticEnv);

src/libexpr/nixexpr.hh

@@ -16,6 +16,7 @@ MakeError(ThrownError, AssertionError)
 MakeError(Abort, EvalError)
 MakeError(TypeError, EvalError)
 MakeError(UndefinedVarError, Error)
+MakeError(RestrictedPathError, Error)
 
 
 /* Position objects. */

src/libexpr/parser.y

@@ -614,7 +614,8 @@ void EvalState::addToSearchPath(const string & s, bool warn)
     path = absPath(path);
     if (pathExists(path)) {
         debug(format("adding path ‘%1%’ to the search path") % path);
-        searchPath.push_back(std::pair<string, Path>(prefix, path));
+        /* Resolve symlinks in the path to support restricted mode. */
+        searchPath.push_back(std::pair<string, Path>(prefix, canonPath(path, true)));
     } else if (warn)
         printMsg(lvlError, format("warning: Nix search path entry ‘%1%’ does not exist, ignoring") % path);
 }

src/libexpr/primops.cc

@@ -80,6 +80,8 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
             % path % e.path % pos);
     }
 
+    path = state.checkSourcePath(path);
+
     if (isStorePath(path) && store->isValidPath(path) && isDerivation(path)) {
         Derivation drv = readDerivation(path);
         Value & w = *state.allocValue();
@@ -133,7 +135,7 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
 /* !!! Should we pass the Pos or the file name too? */
 extern "C" typedef void (*ValueInitializer)(EvalState & state, Value & v);
 
-/* Load a ValueInitializer from a dso and return whatever it initializes */
+/* Load a ValueInitializer from a DSO and return whatever it initializes */
 static void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
@@ -146,6 +148,8 @@ static void prim_importNative(EvalState & state, const Pos & pos, Value * * args
             % path % e.path % pos);
     }
 
+    path = state.checkSourcePath(path);
+
     string sym = state.forceStringNoCtx(*args[1], pos);
 
     void *handle = dlopen(path.c_str(), RTLD_LAZY | RTLD_LOCAL);
@@ -380,7 +384,7 @@ static void prim_tryEval(EvalState & state, const Pos & pos, Value * * args, Val
 static void prim_getEnv(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     string name = state.forceStringNoCtx(*args[0], pos);
-    mkString(v, getEnv(name));
+    mkString(v, state.restricted ? "" : getEnv(name));
 }
 
 
@@ -680,7 +684,7 @@ static void prim_toPath(EvalState & state, const Pos & pos, Value * * args, Valu
 static void prim_storePath(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
-    Path path = state.coerceToPath(pos, *args[0], context);
+    Path path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context));
     /* Resolve symlinks in ‘path’, unless ‘path’ itself is a symlink
        directly in the store.  The latter condition is necessary so
        e.g. nix-push does the right thing. */
@@ -701,7 +705,15 @@ static void prim_pathExists(EvalState & state, const Pos & pos, Value * * args,
     Path path = state.coerceToPath(pos, *args[0], context);
     if (!context.empty())
         throw EvalError(format("string ‘%1%’ cannot refer to other paths, at %2%") % path % pos);
-    mkBool(v, pathExists(path));
+    try {
+        mkBool(v, pathExists(state.checkSourcePath(path)));
+    } catch (SysError & e) {
+        /* Don't give away info from errors while canonicalising
+           ‘path’ in restricted mode. */
+        mkBool(v, false);
+    } catch (RestrictedPathError & e) {
+        mkBool(v, false);
+    }
 }
 
 
@@ -736,7 +748,7 @@ static void prim_readFile(EvalState & state, const Pos & pos, Value * * args, Va
         throw EvalError(format("cannot read ‘%1%’, since path ‘%2%’ is not valid, at %3%")
             % path % e.path % pos);
     }
-    mkString(v, readFile(path).c_str());
+    mkString(v, readFile(state.checkSourcePath(path)).c_str());
 }
 
 
@@ -763,7 +775,7 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
             throw EvalError(format("attribute ‘path’ missing, at %1%") % pos);
         string path = state.coerceToPath(pos, *i->value, context);
 
-        searchPath.push_back(std::pair<string, Path>(prefix, path));
+        searchPath.push_back(std::pair<string, Path>(prefix, state.checkSourcePath(path)));
     }
 
     string path = state.forceStringNoCtx(*args[1], pos);
@@ -790,7 +802,7 @@ static void prim_readDir(EvalState & state, const Pos & pos, Value * * args, Val
             % path % e.path % pos);
     }
 
-    DirEntries entries = readDirectory(path);
+    DirEntries entries = readDirectory(state.checkSourcePath(path));
     state.mkAttrs(v, entries.size());
 
     for (auto & ent : entries) {
@@ -927,6 +939,8 @@ static void prim_filterSource(EvalState & state, const Pos & pos, Value * * args
 
     FilterFromExpr filter(state, *args[0]);
 
+    path = state.checkSourcePath(path);
+
     Path dstPath = settings.readOnlyMode
         ? computeStorePathForPath(path, true, htSHA256, filter).first
         : store->addToStore(path, true, htSHA256, filter, state.repair);