Add check for file system access to the sandbox.

sandbox/linux/services/credentials.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sandbox/linux/services/credentials.h"
 
 #include <errno.h>
 #include <limits.h>
 #include <signal.h>
 #include <stddef.h>
@@ skipping 297 matching lines @@
   PCHECK(NamespaceUtils::WriteToIdMapFile(kGidMapFile, gid));
   PCHECK(NamespaceUtils::WriteToIdMapFile(kUidMapFile, uid));
   DCHECK(GetRESIds(NULL, NULL));
   return true;
 }
 
 bool Credentials::DropFileSystemAccess(int proc_fd) {
   CHECK_LE(0, proc_fd);
 
   CHECK(ChrootToSafeEmptyDir());
   CHECK(!base::DirectoryExists(base::FilePath("/proc")));

[rickyz (no longer on Chrome), 2016/09/23 23:40:20]

Mind replacing this line with CHECK(!HasFileSystemAccess())

[Greg K, 2016/09/26 20:22:12]

Done.

   CHECK(!ProcUtil::HasOpenDirectory(proc_fd));
   // We never let this function fail.
   return true;
 }
 
+bool Credentials::HasFileSystemAccess() {
+  return base::DirectoryExists(base::FilePath("/proc"));
+}
+
 pid_t Credentials::ForkAndDropCapabilitiesInChild() {
   pid_t pid = fork();
   if (pid != 0) {
     return pid;
   }
 
   // Since we just forked, we are single threaded.
   PCHECK(DropAllCapabilitiesOnCurrentThread());
   return 0;
 }
 
 }  // namespace sandbox.