Re: [Virtio-fs] [PATCH] docs: describe the security considerations with virtiofsd xattr mapping

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Jun 11, 2021 at 01:04:27PM +0100, Daniel P. Berrangé 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 <berrange@redhat.com>
> ---
>  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.

Remapping all xattrs seem to make sense. It probably will lead
to less confusion. Nested guests add another level of redirection.

BTW, remapping xattr has limitation that it does not work on
symlinks. So "user.*" can't be set on symlink. And that means
selinux relabeling of symlinks fails with remapped xattrs.

Not sure how to address this limitation. Host kernel imposes
this limit. (man xattr).

> +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.

Example (2) seems to block all the xattrs with prefix even
if only one xattr has been remapped.

So if we remapped "trusted." to "user.virtiofs.trusted.", then
client can't set any xattr starting with "user.virtiofs". I am
wondering should it be limted to only blocking only
"user.virtiofs.trusted.".

Thanks
Vivek

> +
> +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
> 
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