Fix userns_enabled? check for unprivileged_userns_clone

[bcoles]

The unprivileged_userns_clone sysctl parameter may not be present on the target system.

On Fedora 28 Workstation, /proc/sys/kernel/unprivileged_userns_clone is not present (nor is userns_restrict), and the system fails open in its absence, allowing unprivileged users to create user namespaces.

Prior to this patch, the userns_enabled? method would falsely report unprivileged user namespaces as disabled in the event that the unprivileged_userns_clone sysctl parameter was not present.

The Boolean check for a value of 1 was causing the userns_enabled? method to incorrectly return false, as cat /proc/sys/kernel/unprivileged_userns_clone was returning cat: /proc/sys/kernel/unprivileged_userns_clone: No such file or directory.

---

diff --git a/lib/msf/core/post/linux/kernel.rb b/lib/msf/core/post/linux/kernel.rb
index 4b6faf6..831a2f6 100644
--- a/lib/msf/core/post/linux/kernel.rb
+++ b/lib/msf/core/post/linux/kernel.rb
@@ -105,7 +105,8 @@ module Kernel
   #
   def userns_enabled?
     return false if cmd_exec('cat /proc/sys/user/max_user_namespaces').to_s.strip.eql? '0'
-    cmd_exec('cat /proc/sys/kernel/unprivileged_userns_clone').to_s.strip.eql? '1'
+    return false if cmd_exec('cat /proc/sys/kernel/unprivileged_userns_clone').to_s.strip.eql? '0'
+    true
   rescue
     raise 'Could not determine userns status'
   end

[busterb]

LGTM, thanks @bcoles

[busterb]

Release Notes

This fixes the userns_enabled? method for Linux kernel post module capability detection when the unprivileged_userns_clone or proc file entries are unavailable.