Patch: Add support for chroot to Worker#user

Patch: Add support for chroot to Worker#user

[Jeremy Evans]

Any chrooting would need to happen inside Worker#user, because
you can't chroot until after you have parsed the list of groups,
and you must chroot before dropping root privileges.

chroot is an important security feature, so that if the unicorn
process is exploited, file system access is limited to the chroot
directory instead of the entire system.

This is not a complete patch as it does not include tests. I can
add tests if you would consider accepting this feature.
---
 lib/unicorn/worker.rb | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/lib/unicorn/worker.rb b/lib/unicorn/worker.rb
index 6748a2f..db418ea 100644
--- a/lib/unicorn/worker.rb
+++ b/lib/unicorn/worker.rb
@@ -111,9 +111,10 @@ def close # :nodoc:
   # In most cases, you should be using the Unicorn::Configurator#user
   # directive instead.  This method should only be used if you need
   # fine-grained control of exactly when you want to change permissions
-  # in your after_fork hooks.
+  # in your after_fork hooks, or if you want to use the chroot support.
   #
-  # Changes the worker process to the specified +user+ and +group+
+  # Changes the worker process to the specified +user+ and +group+,
+  # and chroots to the current working directory if +chroot+ is set.
   # This is only intended to be called from within the worker
   # process from the +after_fork+ hook.  This should be called in
   # the +after_fork+ hook after any privileged functions need to be
@@ -123,7 +124,7 @@ def close # :nodoc:
   # directly back to the caller (usually the +after_fork+ hook.
   # These errors commonly include ArgumentError for specifying an
   # invalid user/group and Errno::EPERM for insufficient privileges
-  def user(user, group = nil)
+  def user(user, group = nil, chroot = false)
     # we do not protect the caller, checking Process.euid == 0 is
     # insufficient because modern systems have fine-grained
     # capabilities.  Let the caller handle any and all errors.
@@ -134,6 +135,10 @@ def user(user, group = nil)
       Process.initgroups(user, gid)
       Process::GID.change_privilege(gid)
     end
+    if chroot
+      Dir.chroot(Dir.pwd)
+      Dir.chdir('/')
+    end
     Process.euid != uid and Process::UID.change_privilege(uid)
     @switched = true
   end
-- 
2.11.0

Re: Patch: Add support for chroot to Worker#user

[Eric Wong]

Jeremy Evans <code@jeremyevans.net> wrote:
> Any chrooting would need to happen inside Worker#user, because
> you can't chroot until after you have parsed the list of groups,
> and you must chroot before dropping root privileges.
> 
> chroot is an important security feature, so that if the unicorn
> process is exploited, file system access is limited to the chroot
> directory instead of the entire system.

I'm hesitant to document this as "an important security feature".

Perhaps "an extra layer of security" is more accurate,
as there are ways of breaking out of a chroot.

> This is not a complete patch as it does not include tests. I can
> add tests if you would consider accepting this feature.

I wouldn't worry about automated tests if elevated privileges
are required.

> -  # Changes the worker process to the specified +user+ and +group+
> +  # Changes the worker process to the specified +user+ and +group+,
> +  # and chroots to the current working directory if +chroot+ is set.
>    # This is only intended to be called from within the worker
>    # process from the +after_fork+ hook.  This should be called in
>    # the +after_fork+ hook after any privileged functions need to be
> @@ -123,7 +124,7 @@ def close # :nodoc:
>    # directly back to the caller (usually the +after_fork+ hook.
>    # These errors commonly include ArgumentError for specifying an
>    # invalid user/group and Errno::EPERM for insufficient privileges
> -  def user(user, group = nil)
> +  def user(user, group = nil, chroot = false)
>      # we do not protect the caller, checking Process.euid == 0 is
>      # insufficient because modern systems have fine-grained
>      # capabilities.  Let the caller handle any and all errors.
> @@ -134,6 +135,10 @@ def user(user, group = nil)
>        Process.initgroups(user, gid)
>        Process::GID.change_privilege(gid)
>      end
> +    if chroot
> +      Dir.chroot(Dir.pwd)
> +      Dir.chdir('/')
> +    end

Perhaps this can be made more flexible by allowing chroot to
any specified directory, not just pwd.  Something like:

    chroot = Dir.pwd if chroot == true
    if chroot
      Dir.chroot(chroot)
      Dir.chdir('/')
    end

Anyways I'm inclined to accept this feature.

Re: Patch: Add support for chroot to Worker#user

[Jeremy Evans]

On 02/21 07:53, Eric Wong wrote:
> Jeremy Evans <code@jeremyevans.net> wrote:
> > Any chrooting would need to happen inside Worker#user, because
> > you can't chroot until after you have parsed the list of groups,
> > and you must chroot before dropping root privileges.
> > 
> > chroot is an important security feature, so that if the unicorn
> > process is exploited, file system access is limited to the chroot
> > directory instead of the entire system.
> 
> I'm hesitant to document this as "an important security feature".
> 
> Perhaps "an extra layer of security" is more accurate,
> as there are ways of breaking out of a chroot.

That's fine.  Note that while breaking out of a chroot is easy if you
have root privileges, it is difficult to break out of a chroot
if root privileges have been dropped.  There are a couple of
possibilities listed at https://github.com/earthquake/chw00t, neither of
which is likely in environments where unicorn is typically deployed.

> 
> > This is not a complete patch as it does not include tests. I can
> > add tests if you would consider accepting this feature.
> 
> I wouldn't worry about automated tests if elevated privileges
> are required.
> 
> > -  # Changes the worker process to the specified +user+ and +group+
> > +  # Changes the worker process to the specified +user+ and +group+,
> > +  # and chroots to the current working directory if +chroot+ is set.
> >    # This is only intended to be called from within the worker
> >    # process from the +after_fork+ hook.  This should be called in
> >    # the +after_fork+ hook after any privileged functions need to be
> > @@ -123,7 +124,7 @@ def close # :nodoc:
> >    # directly back to the caller (usually the +after_fork+ hook.
> >    # These errors commonly include ArgumentError for specifying an
> >    # invalid user/group and Errno::EPERM for insufficient privileges
> > -  def user(user, group = nil)
> > +  def user(user, group = nil, chroot = false)
> >      # we do not protect the caller, checking Process.euid == 0 is
> >      # insufficient because modern systems have fine-grained
> >      # capabilities.  Let the caller handle any and all errors.
> > @@ -134,6 +135,10 @@ def user(user, group = nil)
> >        Process.initgroups(user, gid)
> >        Process::GID.change_privilege(gid)
> >      end
> > +    if chroot
> > +      Dir.chroot(Dir.pwd)
> > +      Dir.chdir('/')
> > +    end
> 
> Perhaps this can be made more flexible by allowing chroot to
> any specified directory, not just pwd.  Something like:
> 
>     chroot = Dir.pwd if chroot == true
>     if chroot
>       Dir.chroot(chroot)
>       Dir.chdir('/')
>     end
> 
> Anyways I'm inclined to accept this feature.

I had basically the same code originally, but wanted to minimize the API
surface to increase the likelihood for acceptance.  So I'm definitely in
favor of that change.

Thanks,
Jeremy