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| -rw-r--r-- | docs/tools/virtiofsd.rst | 55 |
1 files changed, 50 insertions, 5 deletions
diff --git a/docs/tools/virtiofsd.rst b/docs/tools/virtiofsd.rst index 4911e79..a6c3502 100644 --- a/docs/tools/virtiofsd.rst +++ b/docs/tools/virtiofsd.rst @@ -127,8 +127,8 @@ Options timeout. ``always`` sets a long cache lifetime at the expense of coherency. The default is ``auto``. -xattr-mapping -------------- +Extended attribute (xattr) mapping +---------------------------------- By default the name of xattr's used by the client are passed through to the server file system. This can be a problem where either those xattr names are used @@ -136,6 +136,9 @@ by something on the server (e.g. selinux client/server confusion) or if the virtiofsd is running in a container with restricted privileges where it cannot access some attributes. +Mapping syntax +~~~~~~~~~~~~~~ + A mapping of xattr names can be made using -o xattrmap=mapping where the ``mapping`` string consists of a series of rules. @@ -232,8 +235,48 @@ Note: When the 'security.capability' xattr is remapped, the daemon has to do extra work to remove it during many operations, which the host kernel normally does itself. -xattr-mapping Examples ----------------------- +Security considerations +~~~~~~~~~~~~~~~~~~~~~~~ + +Operating systems typically partition the xattr namespace using +well defined name prefixes. Each partition may have different +access controls applied. For example, on Linux there are multiple +partitions + + * ``system.*`` - access varies depending on attribute & filesystem + * ``security.*`` - only processes with CAP_SYS_ADMIN + * ``trusted.*`` - only processes with CAP_SYS_ADMIN + * ``user.*`` - any process granted by file permissions / ownership + +While other OS such as FreeBSD have different name prefixes +and access control rules. + +When remapping attributes on the host, it is important to +ensure that the remapping does not allow a guest user to +evade the guest access control rules. + +Consider if ``trusted.*`` from the guest was remapped to +``user.virtiofs.trusted*`` in the host. An unprivileged +user in a Linux guest has the ability to write to xattrs +under ``user.*``. Thus the user can evade the access +control restriction on ``trusted.*`` by instead writing +to ``user.virtiofs.trusted.*``. + +As noted above, the partitions used and access controls +applied, will vary across guest OS, so it is not wise to +try to predict what the guest OS will use. + +The simplest way to avoid an insecure configuration is +to remap all xattrs at once, to a given fixed prefix. +This is shown in example (1) below. + +If selectively mapping only a subset of xattr prefixes, +then rules must be added to explicitly block direct +access to the target of the remapping. This is shown +in example (2) below. + +Mapping examples +~~~~~~~~~~~~~~~~ 1) Prefix all attributes with 'user.virtiofs.' @@ -271,7 +314,9 @@ stripping of 'user.virtiofs.'. The second rule hides unprefixed 'trusted.' attributes on the host. The third rule stops a guest from explicitly setting -the 'user.virtiofs.' path directly. +the 'user.virtiofs.' path directly to prevent access +control bypass on the target of the earlier prefix +remapping. Finally, the fourth rule lets all remaining attributes through. |