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04. Required Modifications

This chapter will describe the essential Kernel modifications to implement something like Linux-VServer.

04.0. Context Separation

The separation mentioned in the Concepts section requires some modifications to the kernel to allow for the notion of Contexts.

The purpose of this "Context" is to hide all processes outside of its scope, and prohibit any unwanted interaction between a process inside the context and a process belonging to another context.

This separation requires the extension of some existing data structures in order for them to become aware of contexts and to differentiate between identical uids used in different virtual servers.

It also requires the definition of a default context that is used when the host system is booted, and to work around the issues resulting from some false assumptions made by some user-space tools (like pstree) that the init process has to exist and to be running under id '1'.

To simplify administration, the Host Context isn't treated any differently than any other context as far as process isolation is concerned. To allow for process overview, a special Spectator context has been defined to peek at all processes at once.

See Examples [E21], [E22] and [E23]

04.1. Network Separation

While the Context Separation is sufficient to isolate groups of processes, a different kind of separation, or rather a limitation, is required to confine processes to a subset of available network addresses.

Several issues have to be considered when doing so; for example, the fact that bindings to special addresses like IPADDR_ANY or the local host address have to be handled in a very special way.

Currently, Linux-VServer doesn't make use of virtual network devices (and maybe never will) to minimize the resulting overhead. Therefore socket binding and packet transmission have been adjusted.

See Example [E24]

04.2. The Chroot Barrier

One major problem of the chroot() system used in Linux lies within the fact that this information is volatile, and will be changed on the next chroot() Syscall.

One simple method to escape from a chroot-ed environment is as follows: First, create or open a file and retain the file-descriptor, then chroot into a subdirectory at equal or lower level with regards to the file. This causes the root to be moved down in the filesystem. Next, use fchdir() on the file-descriptor to escape from that new root. This will consequently escape from the old root as well, as this was lost in the last chroot() Syscall.

While early Linux-VServer versions tried to fix this by "funny" methods, recent versions use a special marking, known as the Chroot Barrier, on the parent directory of each VPS to prevent unauthorized modification and escape from confinement.

04.3. Upper Bound for Caps

Because the current Linux Capability system does not implement the filesystem related portions of POSIX Capabilities which would make setuid and setgid executables secure, and because it is much safer to have a secure upper bound for all processes within a context, an additional per-context capability mask has been added to limit all processes belonging to that context to this mask.

The meaning of the individual caps (bits) of the capability bound mask is exactly the same as with the permitted capability set.

04.4. Resource Isolation

Most resources are somewhat shared among the different contexts; some require additional Isolation than others, either to avoid security issues or to allow for improved accounting.

Those resources are:

04.5. Filesystem XID Tagging

Although it can be disabled completely, this modification is required for more robust filesystem level security and context isolation. It also is mandatory for Context Disk Limits and Per Context Quota Support on a shared partition.

The concept of adding a context id (xid) to each file to make the context ownership persistent sounds simple, but the actual implementation is non-trivial - mainly because adding this information either requires a change to the on disk representation of the filesystem or the application of some tricks.

One non-intrusive approach to avoid modification of the underlying filesystem is to use the upper bits of existing fields, like those for UID and GID to store the additional XID.

Once context information is available for each inode, it is a logical step to extend the access controls to check against context too.

Currently all inode access restrictions have been extended to check for the context id, with special exceptions for the Host Context and the Spectator Context.

Untagged files belong to the Host Context and are silently treated as if they belong to the current context, which is required for Unification. If such a file is modified from inside a context, it silently migrates to the new one, changing its xid.

The following Tagging Methods are implemented:

This format uses currently unused space within the disk inode to store the context information. As of now, this is only defined for ext2/ext3 but will be also defined for xfs, reiserfs, and jfs as soon as possible. Advantage: Full 32bit uid/gid values.
This format uses the upper half of the group id to store the context information. This is done transparently, except if the format is changed without prior file conversion. Advantage: works on all 32bit U/GID FSs. Drawback: GID is reduced to 16 bits.
This format uses the upper quarter of user and group id to store the context information, again transparently. This allows for about 16 million user and group ids, which should suffice for the majority of all applications. Advantage: works on all 32bit U/GID FSs. Drawback: UID and GID are reduced to 24 bits.

See Examples [E31] and [E32]

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