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Diffstat (limited to 'docs/tools/virtiofsd.rst')
| -rw-r--r-- | docs/tools/virtiofsd.rst | 193 |
1 files changed, 186 insertions, 7 deletions
diff --git a/docs/tools/virtiofsd.rst b/docs/tools/virtiofsd.rst index 7ecee49834..34a9e40146 100644 --- a/docs/tools/virtiofsd.rst +++ b/docs/tools/virtiofsd.rst @@ -17,13 +17,24 @@ This program is designed to work with QEMU's ``--device vhost-user-fs-pci`` but should work with any virtual machine monitor (VMM) that supports vhost-user. See the Examples section below. -This program must be run as the root user. Upon startup the program will -switch into a new file system namespace with the shared directory tree as its -root. This prevents "file system escapes" due to symlinks and other file -system objects that might lead to files outside the shared directory. The -program also sandboxes itself using seccomp(2) to prevent ptrace(2) and other -vectors that could allow an attacker to compromise the system after gaining -control of the virtiofsd process. +This program must be run as the root user. The program drops privileges where +possible during startup although it must be able to create and access files +with any uid/gid: + +* The ability to invoke syscalls is limited using seccomp(2). +* Linux capabilities(7) are dropped. + +In "namespace" sandbox mode the program switches into a new file system +namespace and invokes pivot_root(2) to make the shared directory tree its root. +A new pid and net namespace is also created to isolate the process. + +In "chroot" sandbox mode the program invokes chroot(2) to make the shared +directory tree its root. This mode is intended for container environments where +the container runtime has already set up the namespaces and the program does +not have permission to create namespaces itself. + +Both sandbox modes prevent "file system escapes" due to symlinks and other file +system objects that might lead to files outside the shared directory. Options ------- @@ -69,6 +80,13 @@ Options * readdirplus|no_readdirplus - Enable/disable readdirplus. The default is ``readdirplus``. + * sandbox=namespace|chroot - + Sandbox mode: + - namespace: Create mount, pid, and net namespaces and pivot_root(2) into + the shared directory. + - chroot: chroot(2) into shared directory (use in containers). + The default is "namespace". + * source=PATH - Share host directory tree located at PATH. This option is required. @@ -109,6 +127,167 @@ Options timeout. ``always`` sets a long cache lifetime at the expense of coherency. The default is ``auto``. +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 +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. + +A mapping of xattr names can be made using -o xattrmap=mapping where the ``mapping`` +string consists of a series of rules. + +The first matching rule terminates the mapping. +The set of rules must include a terminating rule to match any remaining attributes +at the end. + +Each rule consists of a number of fields separated with a separator that is the +first non-white space character in the rule. This separator must then be used +for the whole rule. +White space may be added before and after each rule. + +Using ':' as the separator a rule is of the form: + +``:type:scope:key:prepend:`` + +**scope** is: + +- 'client' - match 'key' against a xattr name from the client for + setxattr/getxattr/removexattr +- 'server' - match 'prepend' against a xattr name from the server + for listxattr +- 'all' - can be used to make a single rule where both the server + and client matches are triggered. + +**type** is one of: + +- 'prefix' - is designed to prepend and strip a prefix; the modified + attributes then being passed on to the client/server. + +- 'ok' - Causes the rule set to be terminated when a match is found + while allowing matching xattr's through unchanged. + It is intended both as a way of explicitly terminating + the list of rules, and to allow some xattr's to skip following rules. + +- 'bad' - If a client tries to use a name matching 'key' it's + denied using EPERM; when the server passes an attribute + name matching 'prepend' it's hidden. In many ways it's use is very like + 'ok' as either an explict terminator or for special handling of certain + patterns. + +**key** is a string tested as a prefix on an attribute name originating +on the client. It maybe empty in which case a 'client' rule +will always match on client names. + +**prepend** is a string tested as a prefix on an attribute name originating +on the server, and used as a new prefix. It may be empty +in which case a 'server' rule will always match on all names from +the server. + +e.g.: + + ``:prefix:client:trusted.:user.virtiofs.:`` + + will match 'trusted.' attributes in client calls and prefix them before + passing them to the server. + + ``:prefix:server::user.virtiofs.:`` + + will strip 'user.virtiofs.' from all server replies. + + ``:prefix:all:trusted.:user.virtiofs.:`` + + combines the previous two cases into a single rule. + + ``:ok:client:user.::`` + + will allow get/set xattr for 'user.' xattr's and ignore + following rules. + + ``:ok:server::security.:`` + + will pass 'securty.' xattr's in listxattr from the server + and ignore following rules. + + ``:ok:all:::`` + + will terminate the rule search passing any remaining attributes + in both directions. + + ``:bad:server::security.:`` + + would hide 'security.' xattr's in listxattr from the server. + +A simpler 'map' type provides a shorter syntax for the common case: + +``:map:key:prepend:`` + +The 'map' type adds a number of separate rules to add **prepend** as a prefix +to the matched **key** (or all attributes if **key** is empty). +There may be at most one 'map' rule and it must be the last rule in the set. + +xattr-mapping Examples +---------------------- + +1) Prefix all attributes with 'user.virtiofs.' + +:: + +-o xattrmap=":prefix:all::user.virtiofs.::bad:all:::" + + +This uses two rules, using : as the field separator; +the first rule prefixes and strips 'user.virtiofs.', +the second rule hides any non-prefixed attributes that +the host set. + +This is equivalent to the 'map' rule: + +:: +-o xattrmap=":map::user.virtiofs.:" + +2) Prefix 'trusted.' attributes, allow others through + +:: + + "/prefix/all/trusted./user.virtiofs./ + /bad/server//trusted./ + /bad/client/user.virtiofs.// + /ok/all///" + + +Here there are four rules, using / as the field +separator, and also demonstrating that new lines can +be included between rules. +The first rule is the prefixing of 'trusted.' and +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. +Finally, the fourth rule lets all remaining attributes +through. + +This is equivalent to the 'map' rule: + +:: +-o xattrmap="/map/trusted./user.virtiofs./" + +3) Hide 'security.' attributes, and allow everything else + +:: + + "/bad/all/security./security./ + /ok/all///' + +The first rule combines what could be separate client and server +rules into a single 'all' rule, matching 'security.' in either +client arguments or lists returned from the host. This stops +the client seeing any 'security.' attributes on the server and +stops it setting any. + Examples -------- |