16b0568d2a
By default, all access to the upper, lower and work directories is the recorded mounter's MAC and DAC credentials. The incoming accesses are checked against the caller's credentials. If the principles of least privilege are applied, the mounter's credentials might not overlap the credentials of the caller's when accessing the overlayfs filesystem. For example, a file that a lower DAC privileged caller can execute, is MAC denied to the generally higher DAC privileged mounter, to prevent an attack vector. We add the option to turn off override_creds in the mount options; all subsequent operations after mount on the filesystem will be only the caller's credentials. The module boolean parameter and mount option override_creds is also added as a presence check for this "feature", existence of /sys/module/overlay/parameters/override_creds. It was not always this way. Circa 4.6 there was no recorded mounter's credentials, instead privileged access to upper or work directories were temporarily increased to perform the operations. The MAC (selinux) policies were caller's in all cases. override_creds=off partially returns us to this older access model minus the insecure temporary credential increases. This is to permit use in a system with non-overlapping security models for each executable including the agent that mounts the overlayfs filesystem. In Android this is the case since init, which performs the mount operations, has a minimal MAC set of privileges to reduce any attack surface, and services that use the content have a different set of MAC privileges (eg: read, for vendor labelled configuration, execute for vendor libraries and modules). The caveats are not a problem in the Android usage model, however they should be fixed for completeness and for general use in time. Signed-off-by: Mark Salyzyn <salyzyn@android.com> Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Vivek Goyal <vgoyal@redhat.com> Cc: Eric W. Biederman <ebiederm@xmission.com> Cc: Amir Goldstein <amir73il@gmail.com> Cc: Randy Dunlap <rdunlap@infradead.org> Cc: Stephen Smalley <sds@tycho.nsa.gov> Cc: linux-unionfs@vger.kernel.org Cc: linux-doc@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: kernel-team@android.com
240 lines
9.5 KiB
Plaintext
240 lines
9.5 KiB
Plaintext
Written by: Neil Brown
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Please see MAINTAINERS file for where to send questions.
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Overlay Filesystem
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==================
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This document describes a prototype for a new approach to providing
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overlay-filesystem functionality in Linux (sometimes referred to as
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union-filesystems). An overlay-filesystem tries to present a
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filesystem which is the result over overlaying one filesystem on top
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of the other.
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The result will inevitably fail to look exactly like a normal
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filesystem for various technical reasons. The expectation is that
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many use cases will be able to ignore these differences.
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This approach is 'hybrid' because the objects that appear in the
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filesystem do not all appear to belong to that filesystem. In many
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cases an object accessed in the union will be indistinguishable
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from accessing the corresponding object from the original filesystem.
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This is most obvious from the 'st_dev' field returned by stat(2).
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While directories will report an st_dev from the overlay-filesystem,
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all non-directory objects will report an st_dev from the lower or
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upper filesystem that is providing the object. Similarly st_ino will
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only be unique when combined with st_dev, and both of these can change
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over the lifetime of a non-directory object. Many applications and
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tools ignore these values and will not be affected.
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Upper and Lower
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---------------
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An overlay filesystem combines two filesystems - an 'upper' filesystem
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and a 'lower' filesystem. When a name exists in both filesystems, the
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object in the 'upper' filesystem is visible while the object in the
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'lower' filesystem is either hidden or, in the case of directories,
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merged with the 'upper' object.
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It would be more correct to refer to an upper and lower 'directory
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tree' rather than 'filesystem' as it is quite possible for both
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directory trees to be in the same filesystem and there is no
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requirement that the root of a filesystem be given for either upper or
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lower.
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The lower filesystem can be any filesystem supported by Linux and does
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not need to be writable. The lower filesystem can even be another
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overlayfs. The upper filesystem will normally be writable and if it
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is it must support the creation of trusted.* extended attributes, and
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must provide valid d_type in readdir responses, so NFS is not suitable.
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A read-only overlay of two read-only filesystems may use any
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filesystem type.
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Directories
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-----------
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Overlaying mainly involves directories. If a given name appears in both
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upper and lower filesystems and refers to a non-directory in either,
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then the lower object is hidden - the name refers only to the upper
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object.
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Where both upper and lower objects are directories, a merged directory
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is formed.
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At mount time, the two directories given as mount options "lowerdir" and
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"upperdir" are combined into a merged directory:
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mount -t overlay overlay -olowerdir=/lower,upperdir=/upper,\
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workdir=/work /merged
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The "workdir" needs to be an empty directory on the same filesystem
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as upperdir.
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Then whenever a lookup is requested in such a merged directory, the
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lookup is performed in each actual directory and the combined result
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is cached in the dentry belonging to the overlay filesystem. If both
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actual lookups find directories, both are stored and a merged
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directory is created, otherwise only one is stored: the upper if it
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exists, else the lower.
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Only the lists of names from directories are merged. Other content
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such as metadata and extended attributes are reported for the upper
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directory only. These attributes of the lower directory are hidden.
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credentials
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-----------
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By default, all access to the upper, lower and work directories is the
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recorded mounter's MAC and DAC credentials. The incoming accesses are
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checked against the caller's credentials.
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In the case where caller MAC or DAC credentials do not overlap, a
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use case available in older versions of the driver, the
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override_creds mount flag can be turned off and help when the use
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pattern has caller with legitimate credentials where the mounter
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does not. Several unintended side effects will occur though. The
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caller without certain key capabilities or lower privilege will not
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always be able to delete files or directories, create nodes, or
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search some restricted directories. The ability to search and read
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a directory entry is spotty as a result of the cache mechanism not
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retesting the credentials because of the assumption, a privileged
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caller can fill cache, then a lower privilege can read the directory
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cache. The uneven security model where cache, upperdir and workdir
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are opened at privilege, but accessed without creating a form of
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privilege escalation, should only be used with strict understanding
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of the side effects and of the security policies.
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whiteouts and opaque directories
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--------------------------------
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In order to support rm and rmdir without changing the lower
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filesystem, an overlay filesystem needs to record in the upper filesystem
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that files have been removed. This is done using whiteouts and opaque
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directories (non-directories are always opaque).
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A whiteout is created as a character device with 0/0 device number.
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When a whiteout is found in the upper level of a merged directory, any
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matching name in the lower level is ignored, and the whiteout itself
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is also hidden.
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A directory is made opaque by setting the xattr "trusted.overlay.opaque"
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to "y". Where the upper filesystem contains an opaque directory, any
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directory in the lower filesystem with the same name is ignored.
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readdir
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-------
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When a 'readdir' request is made on a merged directory, the upper and
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lower directories are each read and the name lists merged in the
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obvious way (upper is read first, then lower - entries that already
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exist are not re-added). This merged name list is cached in the
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'struct file' and so remains as long as the file is kept open. If the
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directory is opened and read by two processes at the same time, they
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will each have separate caches. A seekdir to the start of the
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directory (offset 0) followed by a readdir will cause the cache to be
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discarded and rebuilt.
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This means that changes to the merged directory do not appear while a
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directory is being read. This is unlikely to be noticed by many
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programs.
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seek offsets are assigned sequentially when the directories are read.
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Thus if
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- read part of a directory
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- remember an offset, and close the directory
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- re-open the directory some time later
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- seek to the remembered offset
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there may be little correlation between the old and new locations in
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the list of filenames, particularly if anything has changed in the
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directory.
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Readdir on directories that are not merged is simply handled by the
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underlying directory (upper or lower).
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Non-directories
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---------------
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Objects that are not directories (files, symlinks, device-special
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files etc.) are presented either from the upper or lower filesystem as
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appropriate. When a file in the lower filesystem is accessed in a way
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the requires write-access, such as opening for write access, changing
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some metadata etc., the file is first copied from the lower filesystem
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to the upper filesystem (copy_up). Note that creating a hard-link
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also requires copy_up, though of course creation of a symlink does
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not.
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The copy_up may turn out to be unnecessary, for example if the file is
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opened for read-write but the data is not modified.
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The copy_up process first makes sure that the containing directory
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exists in the upper filesystem - creating it and any parents as
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necessary. It then creates the object with the same metadata (owner,
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mode, mtime, symlink-target etc.) and then if the object is a file, the
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data is copied from the lower to the upper filesystem. Finally any
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extended attributes are copied up.
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Once the copy_up is complete, the overlay filesystem simply
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provides direct access to the newly created file in the upper
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filesystem - future operations on the file are barely noticed by the
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overlay filesystem (though an operation on the name of the file such as
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rename or unlink will of course be noticed and handled).
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Multiple lower layers
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---------------------
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Multiple lower layers can now be given using the the colon (":") as a
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separator character between the directory names. For example:
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mount -t overlay overlay -olowerdir=/lower1:/lower2:/lower3 /merged
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As the example shows, "upperdir=" and "workdir=" may be omitted. In
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that case the overlay will be read-only.
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The specified lower directories will be stacked beginning from the
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rightmost one and going left. In the above example lower1 will be the
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top, lower2 the middle and lower3 the bottom layer.
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Non-standard behavior
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---------------------
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The copy_up operation essentially creates a new, identical file and
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moves it over to the old name. The new file may be on a different
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filesystem, so both st_dev and st_ino of the file may change.
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Any open files referring to this inode will access the old data.
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Any file locks (and leases) obtained before copy_up will not apply
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to the copied up file.
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If a file with multiple hard links is copied up, then this will
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"break" the link. Changes will not be propagated to other names
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referring to the same inode.
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Changes to underlying filesystems
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---------------------------------
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Offline changes, when the overlay is not mounted, are allowed to either
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the upper or the lower trees.
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Changes to the underlying filesystems while part of a mounted overlay
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filesystem are not allowed. If the underlying filesystem is changed,
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the behavior of the overlay is undefined, though it will not result in
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a crash or deadlock.
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Testsuite
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---------
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There's testsuite developed by David Howells at:
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git://git.infradead.org/users/dhowells/unionmount-testsuite.git
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Run as root:
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# cd unionmount-testsuite
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# ./run --ov
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