Red Hat Bugzilla – Bug 527483
/dev/pts must use the 'newinstance' mount flag to avoid security problem with containers
Last modified: 2013-11-06 09:38:38 EST
+++ 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):
Steps to Reproduce:
1. Install a new Fedora host
2. 'mount | grep devpts'
3. ls -l /dev/ptmx
# 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
# 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 comment from email@example.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 firstname.lastname@example.org 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, ...
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
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. **
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.
Latest proposal for upstream kernel is to kill off the 'newinstance' flag and make all /dev/pts instances private by default.
*** Bug 844421 has been marked as a duplicate of this bug. ***
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
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.
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.