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Subject: Re: [virtio-dev] [PATCH v5 09/10] vhost-user: intercept slave's reply to VHOST_USER_GET_PROTOCOL_FEATURES


On Mon, May 18, 2020 at 11:37:20PM +0300, Nikos Dragazis wrote:
> Signed-off-by: Nikos Dragazis <ndragazis@arrikto.com>
> ---
>  virtio-vhost-user.tex | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/virtio-vhost-user.tex b/virtio-vhost-user.tex
> index a673526..328baec 100644
> --- a/virtio-vhost-user.tex
> +++ b/virtio-vhost-user.tex
> @@ -142,10 +142,15 @@ \subsubsection{Device Operation: RX/TX Queues}\label{sec:Device Types / Vhost-us
>  message that is too large for an rxq buffer, then DEVICE_NEEDS_RESET is set and
>  the driver must reset the device.
>  
> -File descriptor passing is handled differently by the vhost-user device
> -backend.  When a message is received that carries one or more file descriptors
> -according to the vhost-user protocol, additional device resources become
> -available to the driver.
> +File descriptor passing is handled differently by the vhost-user device backend.
> +When a master-initiated message is received that carries one or more file
> +descriptors according to the vhost-user protocol, additional device resources
> +become available to the driver.
> +
> +On the contrary, the slave cannot pass file descriptors to the master. For this
> +reason, the vhost-user device backend MUST be intercepting the slave's reply to
> +the VHOST_USER_GET_PROTOCOL_FEATURES vhost-user message and clearing these
> +feature bits that allow the slave to send messages that pass file descriptors.

Please be specific about which vhost-user protocol bits are not
supported.

Why does the virtio-vhost-user device implementation need to silently
clear those feature bits? Is there a security impact or some other
reason why the VIRTIO spec should specify this behavior?

Stefan

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