Enable one PID namespace per process for NaCl processes.

components/nacl/loader/sandbox_linux/nacl_sandbox_linux.cc

 // Copyright 2014 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 "components/nacl/loader/sandbox_linux/nacl_sandbox_linux.h"
 
 #include <errno.h>
 #include <fcntl.h>
 #include <sys/prctl.h>
 #include <sys/stat.h>
@@ skipping 121 matching lines @@
     // the setuid sandbox model.
     CHECK(!HasOpenDirectory());
 
     // Get sandboxed.
     CHECK(setuid_sandbox_client_->ChrootMe());
     CHECK(MaybeSetProcessNonDumpable());
     CHECK(IsSandboxed());
     layer_one_enabled_ = true;
   } else if (sandbox::NamespaceSandbox::InNewUserNamespace()) {
     CHECK(sandbox::Credentials::MoveToNewUserNS());
-    // This relies on SealLayerOneSandbox() to be called later since this

[Mark Seaborn, 2015/06/09 23:44:19]

Isn't this still true?  We still call SealLayerOneSandbox() later and the
requirement is still there, isn't it?

[rickyz (no longer on Chrome), 2015/06/10 01:31:43]

Yeah, it's still true - I removed the comment to be consistent with the fact
that we don't explicitly call out the need for SealLayerOneSandbox when dropping
FS access in the suid sandbox case. The fact that we continue to use proc_fd_
after DropFileSystemAccess hopefully makes it clear that there's still a dir fd
around.

-    // class is keeping a file descriptor to /proc/.
     CHECK(sandbox::Credentials::DropFileSystemAccess(proc_fd_.get()));
-    CHECK(sandbox::Credentials::DropAllCapabilities(proc_fd_.get()));
+
+    // We do not drop CAP_SYS_ADMIN because we need it to place each child
+    // process in its own PID namespace later on.
+    std::vector<sandbox::Credentials::Capability> caps;
+    caps.push_back(sandbox::Credentials::Capability::SYS_ADMIN);
+    CHECK(sandbox::Credentials::SetCapabilities(proc_fd_.get(), caps));
+
     CHECK(IsSandboxed());
     layer_one_enabled_ = true;
   }
 }
 
 void NaClSandbox::CheckForExpectedNumberOfOpenFds() {
   // We expect to have the following FDs open:
   //  1-3) stdin, stdout, stderr.
   //  4) The /dev/urandom FD used by base::GetUrandomFD().
   //  5) A dummy pipe FD used to overwrite kSandboxIPCChannel.
@@ skipping 79 matching lines @@
     static const char kNoBpfMsg[] =
         "The seccomp-bpf sandbox is not engaged for NaCl:";
     if (can_be_no_sandbox)
       LOG(ERROR) << kNoBpfMsg << kItIsDangerousMsg;
     else
       LOG(FATAL) << kNoBpfMsg << kItIsNotAllowedMsg;
   }
 }
 
 }  // namespace nacl