* Daniel P. Berrangé (berra...@redhat.com) wrote: > Different guest xattr prefixes have distinct access control rules applied > by the guest. When remapping a guest xattr care must be taken that the > remapping does not allow the a guest user to bypass guest kernel access > control rules. > > For example if 'trusted.*' which requires CAP_SYS_ADMIN is remapped > to 'user.virtiofs.trusted.*', an unprivileged guest user which can > write to 'user.*' can bypass the CAP_SYS_ADMIN control. Thus the > target of any remapping must be explicitly blocked from read/writes > by the guest, to prevent access control bypass. > > The examples shown in the virtiofsd man page already do the right > thing and ensure safety, but the security implications of getting > this wrong were not made explicit. This could lead to host admins > and apps unwittingly creating insecure configurations. > > Signed-off-by: Daniel P. Berrangé <berra...@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilb...@redhat.com> Vivek's point about symlinks is something we should add but that's kind of separate to the clarification you've explained here. Dave > --- > docs/tools/virtiofsd.rst | 55 ++++++++++++++++++++++++++++++++++++---- > 1 file changed, 50 insertions(+), 5 deletions(-) > > diff --git a/docs/tools/virtiofsd.rst b/docs/tools/virtiofsd.rst > index 00554c75bd..6370ab927c 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.' > > @@ -270,7 +313,9 @@ 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. > > -- > 2.31.1 > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
