FreeBSD Security Advisory - The nullfs(5) filesystem allows all or a part of an already mounted filesystem to be made available in a different part of the global filesystem namespace. It is commonly used to make a set of files available to multiple chroot(2) or jail(2) environments without replicating the files in each environment. A common idiom, described in the FreeBSD Handbook, is to mount one subtree of a filesystem read-only within a jail's filesystem namespace, and mount a different subtree of the same filesystem read-write. The nullfs(5) implementation of the VOP_LINK(9) VFS operation does not check whether the source and target of the link are both in the same nullfs instance. It is therefore possible to create a hardlink from a location in one nullfs instance to a file in another, as long as the underlying (source) filesystem is the same. If multiple nullfs views into the same filesystem are mounted in different locations, a user with read access to one of these views and write access to another will be able to create a hard link from the latter to a file in the former, even though they are, from the user's perspective, different filesystems. The user may thereby gain write access to files which are nominally on a read-only filesystem.
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