Re: [PATCH v3 3/7] vfio/pci: Introduce VF token

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 11 Mar 2020 15:58:52 -0600
Alex Williamson <alex.williamson@redhat.com> wrote:

> If we enable SR-IOV on a vfio-pci owned PF, the resulting VFs are not
> fully isolated from the PF.  The PF can always cause a denial of service
> to the VF, even if by simply resetting itself.  The degree to which a PF
> can access the data passed through a VF or interfere with its operation
> is dependent on a given SR-IOV implementation.  Therefore we want to
> avoid a scenario where an existing vfio-pci based userspace driver might
> assume the PF driver is trusted, for example assigning a PF to one VM
> and VF to another with some expectation of isolation.  IOMMU grouping
> could be a solution to this, but imposes an unnecessarily strong
> relationship between PF and VF drivers if they need to operate with the
> same IOMMU context.  Instead we introduce a "VF token", which is
> essentially just a shared secret between PF and VF drivers, implemented
> as a UUID.
> 
> The VF token can be set by a vfio-pci based PF driver and must be known
> by the vfio-pci based VF driver in order to gain access to the device.
> This allows the degree to which this VF token is considered secret to be
> determined by the applications and environment.  For example a VM might
> generate a random UUID known only internally to the hypervisor while a
> userspace networking appliance might use a shared, or even well know,
> UUID among the application drivers.
> 
> To incorporate this VF token, the VFIO_GROUP_GET_DEVICE_FD interface is
> extended to accept key=value pairs in addition to the device name.  This
> allows us to most easily deny user access to the device without risk
> that existing userspace drivers assume region offsets, IRQs, and other
> device features, leading to more elaborate error paths.  The format of
> these options are expected to take the form:
> 
> "$DEVICE_NAME $OPTION1=$VALUE1 $OPTION2=$VALUE2"
> 
> Where the device name is always provided first for compatibility and
> additional options are specified in a space separated list.  The
> relation between and requirements for the additional options will be
> vfio bus driver dependent, however unknown or unused option within this
> schema should return error.  This allow for future use of unknown
> options as well as a positive indication to the user that an option is
> used.
> 
> An example VF token option would take this form:
> 
> "0000:03:00.0 vf_token=2ab74924-c335-45f4-9b16-8569e5b08258"
> 
> When accessing a VF where the PF is making use of vfio-pci, the user
> MUST provide the current vf_token.  When accessing a PF, the user MUST
> provide the current vf_token IF there are active VF users or MAY provide
> a vf_token in order to set the current VF token when no VF users are
> active.  The former requirement assures VF users that an unassociated
> driver cannot usurp the PF device.  These semantics also imply that a
> VF token MUST be set by a PF driver before VF drivers can access their
> device, the default token is random and mechanisms to read the token are
> not provided in order to protect the VF token of previous users.  Use of
> the vf_token option outside of these cases will return an error, as
> discussed above.
> 
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
>  drivers/vfio/pci/vfio_pci.c         |  198 +++++++++++++++++++++++++++++++++++
>  drivers/vfio/pci/vfio_pci_private.h |    8 +
>  2 files changed, 205 insertions(+), 1 deletion(-)

Reviewed-by: Cornelia Huck <cohuck@redhat.com>