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SubjectRe: [PATCH v3 3/7] vfio/pci: Introduce VF token
On Wed, 11 Mar 2020 15:58:52 -0600
Alex Williamson <> 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:
> 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 <>
> ---
> 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 <>

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