Bug 527483 - /dev/pts must use the 'newinstance' mount flag to avoid security problem with containers
Summary: /dev/pts must use the 'newinstance' mount flag to avoid security problem with...
Alias: None
Product: Red Hat Enterprise Linux 6
Classification: Red Hat
Component: kernel
Version: 6.0
Hardware: All
OS: Linux
Target Milestone: rc
: ---
Assignee: Aristeu Rozanski
QA Contact: Virtualization Bugs
: 844421 (view as bug list)
Depends On:
Blocks: 846704
TreeView+ depends on / blocked
Reported: 2009-10-06 15:20 UTC by Daniel Berrangé
Modified: 2013-11-06 14:38 UTC (History)
3 users (show)

Fixed In Version:
Doc Type: Bug Fix
Doc Text:
Clone Of: 501718
: 860218 (view as bug list)
Last Closed: 2013-11-06 14:38:38 UTC
Target Upstream Version:

Attachments (Terms of Use)

Description Daniel Berrangé 2009-10-06 15:20:36 UTC
+++ This bug was initially created as a clone of Bug #501718 +++

Description of problem:
The Linux kernel now allows multiple instances of 'devpts' filesystem to be mounted on a host, and the pty indexes of each instance are independant. This feature allows Linux containers to be given private 'devpts' instances, preventing them from accessing PTYs from the host OS.

This is only secure, however, if every single mount of 'devpts' uses the 'newinstance' flag, including that of the initial host OS.

If the host OS does not use 'newinstance' mount option, then a container can still get access to the host's PTYs, which is clearly a security problem

When using 'newinstance', the /dev/ptmx  file must also be symlinked, or bind-mounted to /dev/pts/ptmx

There is more information about the required setup in the kernel tree documentation, 'filesystems/devpts.txt'. The key point is in the last paragraph

7. A mount of devpts without the 'newinstance' option results in binding to
   initial kernel mount.  This behavior while preserving legacy semantics,
   does not provide strict isolation in a container environment. i.e by
   mounting devpts without the 'newinstance' option, a container could
   get visibility into the 'host' or root container's devpts.
   To workaround this and have strict isolation, all mounts of devpts,
   including the mount in the root container, should use the newinstance

In Fedora 12, libvirt and the LXC toolchain will both be creating containers using 'newinstance' and want this to provide secure isolation of devpts. Thus we require that the host OS devpts is switched to use 'newinstance' and symlink of /dev/ptmx to /dev/pts/ptmx.

RPM claims the 'setup' RPM owns /etc/fstab, but that seems to be a ghost file, and AFAICT anaconda is responsible for initial creation. Thus I'm filing this bug against anaconda - feel free to reassign if this is the wrong place.

Version-Release number of selected component (if applicable):

How reproducible:

Steps to Reproduce:
1. Install a new Fedora host
2. 'mount | grep devpts'
3. ls -l /dev/ptmx
Actual results:
# mount | grep devpts
devpts on /dev/pts type devpts (rw,gid=5,mode=620)
# ls -l /dev/ptmx
crw-rw-rw-. 1 root tty 5, 2 2009-05-20 14:35 /dev/ptmx

Expected results:
# mount | grep devpts
devpts on /dev/pts type devpts (rw,gid=5,mode=620,newinstance)
# ls -l /dev/ptmx
lrwxrwxrwx. 1 root root 8 2009-05-20 14:36 ptmx -> pts/ptmx

Additional info:

--- Additional comment from berrange@redhat.com on 2009-05-20 09:47:59 EDT ---

Just to clarify, I do not want this for F11. It is too risky to make such a change for F11 at this stage.  Fedora 12 is my desired target for resolution.

--- Additional comment from katzj@redhat.com on 2009-05-20 11:34:24 EDT ---

If we want it mounted that way by default, then we should really change the defaults in the kernel.  Not require changes in anaconda, livecd-creator, appliance-creator, ...

Comment 2 RHEL Program Management 2009-10-06 17:11:31 UTC
This request was evaluated by Red Hat Product Management for inclusion in a Red
Hat Enterprise Linux major release.  Product Management has requested further
review of this request by Red Hat Engineering, for potential inclusion in a Red
Hat Enterprise Linux Major release.  This request is not yet committed for

Comment 3 RHEL Program Management 2010-07-15 14:44:24 UTC
This issue has been proposed when we are only considering blocker
issues in the current Red Hat Enterprise Linux release. It has
been denied for the current Red Hat Enterprise Linux release.

** If you would still like this issue considered for the current
release, ask your support representative to file as a blocker on
your behalf. Otherwise ask that it be considered for the next
Red Hat Enterprise Linux release. **

Comment 4 RHEL Program Management 2011-10-07 14:54:57 UTC
Since RHEL 6.2 External Beta has begun, and this bug remains
unresolved, it has been rejected as it is not proposed as
exception or blocker.

Red Hat invites you to ask your support representative to
propose this request, if appropriate and relevant, in the
next release of Red Hat Enterprise Linux.

Comment 5 Daniel Berrangé 2012-09-25 09:44:12 UTC
Latest proposal for upstream kernel is to kill off the 'newinstance' flag and make all /dev/pts instances private by default.


Comment 6 Daniel Berrangé 2012-09-26 10:36:30 UTC
*** Bug 844421 has been marked as a duplicate of this bug. ***

Comment 7 Aristeu Rozanski 2013-01-21 19:23:29 UTC
FWIW: ebiederman's tree:

also, this might break third party scripts; people remounting devpts in
different directories (think chroot setups) but still wanting to be the same

Comment 8 Daniel Berrangé 2013-01-28 10:21:21 UTC
FYI this change


would potentially offer a different solution. It would allow us to continue issuing a normal devpts in the host. If we want a secure container, then we'd enable user namespaces, which prevent the container accessing the host's devpts since it lacks the newinstance flag.

Comment 9 H. Peter Anvin 2013-02-12 05:42:14 UTC
It isn't really a different solution.  It is just enforcing the new setup in a restricted context.  The change described here is necessary anyway.

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