Security: Improve checking of build products

Build product paths cannot reference locations outside of the Nix
store.  We previously disallowed paths from being symlinks, but this
didn't take into account that parent path elements can be symlinks as
well.  So a build product /nix/store/bla.../foo/passwd, with
/nix/store/bla.../foo being a symlink to /etc, would still work.

So now we check all paths encountered during path resolution.
Symlinks are allowed again so long as they point to the Nix store.

src/lib/Hydra/Controller/Build.pm

@@ -170,10 +170,9 @@ sub defaultUriForProduct {
 
 sub checkPath {
     my ($self, $c, $path) = @_;
-    my $storeDir = $Nix::Config::storeDir . "/";
-    error($c, "Invalid path in build product.")
-        if substr($path, 0, length($storeDir)) ne $storeDir || $path =~ /\/\.\./;
-    error($c, "Path ‘$path’ is a symbolic link.") if -l $path;
+    my $path = pathIsInsidePrefix($path, $Nix::Config::storeDir);
+    error($c, "Build product refers outside of the Nix store.") unless defined $path;
+    return $path;
 }
 
 
@@ -203,7 +202,7 @@ sub download : Chained('build') PathPart {
     $path .= "/" . join("/", @path) if scalar @path > 0;
 
     # Make sure the file is in the Nix store.
-    checkPath($self, $c, $path);
+    $path = checkPath($self, $c, $path);
 
     # If this is a directory but no "/" is attached, then redirect.
     if (-d $path && substr($c->request->uri, -1) ne "/") {
@@ -247,7 +246,7 @@ sub contents : Chained('build') PathPart Args(1) {
 
     my $path = $product->path;
 
-    checkPath($self, $c, $path);
+    $path = checkPath($self, $c, $path);
 
     notFound($c, "Product $path has disappeared.") unless -e $path;
 

src/lib/Hydra/Helper/AddBuilds.pm

@@ -785,15 +785,14 @@ sub addBuildProducts {
                 /^([\w\-]+)\s+([\w\-]+)\s+("[^"]*"|\S+)(\s+(\S+))?$/ or next;
                 my $type = $1;
                 my $subtype = $2 eq "none" ? "" : $2;
-                my $path = File::Spec->canonpath((substr $3, 0, 1) eq "\"" ? substr $3, 1, -1 : $3);
+                my $path = substr($3, 0, 1) eq "\"" ? substr($3, 1, -1) : $3;
                 my $defaultPath = $5;
 
                 # Ensure that the path exists and points into the Nix store.
                 next unless File::Spec->file_name_is_absolute($path);
-                next if $path =~ /\/\.\./; # don't go up
-                next unless substr($path, 0, length($storeDir)) eq $storeDir;
+                $path = pathIsInsidePrefix($path, $Nix::Config::storeDir);
+                next unless defined $path;
                 next unless -e $path;
-                next if -l $path;
 
                 # FIXME: check that the path is in the input closure
                 # of the build?

src/lib/Hydra/Helper/Nix.pm

@@ -16,7 +16,8 @@ our @EXPORT = qw(
     getViewResult getLatestSuccessfulViewResult
     jobsetOverview removeAsciiEscapes getDrvLogPath logContents
     getMainOutput
-    getEvals getMachines);
+    getEvals getMachines
+    pathIsInsidePrefix);
 
 
 sub getHydraHome {
@@ -402,4 +403,45 @@ sub getMachines {
 }
 
 
+# Check whether ‘$path’ is inside ‘$prefix’.  In particular, it checks
+# that resolving symlink components of ‘$path’ never takes us outside
+# of ‘$prefix’.  We use this to check that Nix build products don't
+# refer to things outside of the Nix store (e.g. /etc/passwd) or to
+# symlinks outside of the store that point into the store
+# (e.g. /run/current-system).  Return undef or the resolved path.
+sub pathIsInsidePrefix {
+    my ($path, $prefix) = @_;
+    my $n = 0;
+    $path =~ s/\/+/\//g; # remove redundant slashes
+    $path =~ s/\/*$//; # remove trailing slashes
+
+    return undef unless $path eq $prefix || substr($path, 0, length($prefix) + 1) eq "$prefix/";
+
+    my @cs = File::Spec->splitdir(substr($path, length($prefix) + 1));
+    my $cur = $prefix;
+
+    foreach my $c (@cs) {
+        next if $c eq ".";
+
+        # ‘..’ should not take us outside of the prefix.
+        if ($c eq "..") {
+            return if length($cur) <= length($prefix);
+            $cur =~ s/\/[^\/]*$// or die; # remove last component
+            next;
+        }
+
+        my $new = "$cur/$c";
+        if (-l $new) {
+            my $link = readlink $new or return undef;
+            $new = substr($link, 0, 1) eq "/" ? $link : "$cur/$link";
+            $new = pathIsInsidePrefix($new, $prefix);
+            return undef unless defined $new;
+        }
+        $cur = $new;
+    }
+
+    return $cur;
+}
+
+
 1;