Re: [patch net-next v2] net: virtio_net: implement exact header length guest feature

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Feb 21, 2023 at 04:39:32PM +0100, Jiri Pirko wrote:
> Tue, Feb 21, 2023 at 04:11:53PM CET, willemdebruijn.kernel@gmail.com wrote:
> >Jiri Pirko wrote:
> >> From: Jiri Pirko <jiri@nvidia.com>
> >> 
> >> Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
> >> set implicates that the driver provides the exact size of the header.
> >> 
> >> Quoting the original virtio spec:
> >> "hdr_len is a hint to the device as to how much of the header needs to
> >>  be kept to copy into each packet"
> >> 
> >> "a hint" might not be clear for the reader what does it mean, if it is
> >> "maybe like that" of "exactly like that". This feature just makes it
> >> crystal clear and let the device count on the hdr_len being filled up
> >> by the exact length of header.
> >> 
> >> Also note the spec already has following note about hdr_len:
> >> "Due to various bugs in implementations, this field is not useful
> >>  as a guarantee of the transport header size."
> >> 
> >> Without this feature the device needs to parse the header in core
> >> data path handling. Accurate information helps the device to eliminate
> >> such header parsing and directly use the hardware accelerators
> >> for GSO operation.
> >> 
> >> virtio_net_hdr_from_skb() fills up hdr_len to skb_headlen(skb).
> >> The driver already complies to fill the correct value. Introduce the
> >> feature and advertise it.
> >> 
> >> Note that virtio spec also includes following note for device
> >> implementation:
> >> "Caution should be taken by the implementation so as to prevent
> >>  a malicious driver from attacking the device by setting
> >>  an incorrect hdr_len."
> >> 
> >> There is a plan to support this feature in our emulated device.
> >> A device of SolidRun offers this feature bit. They claim this feature
> >> will save the device a few cycles for every GSO packet.
> >> 
> >> Signed-off-by: Jiri Pirko <jiri@nvidia.com>
> >> ---
> >> v1->v2:
> >> - extended patch description
> >
> >Is the expectation that in-kernel devices support this feature, and
> >if so how would it affect them? If I read the spec correctly, devices
> 
> Well, the tap driver actually trusts the hdr_len to be of correct header
> size nowadays.
> 
> 
> >still need to be careful against malicious drivers, so cannot assume
> >much beyond what they do today (i.e., a hint).
> 
> Malicious how? There is upper limit of size in tap which is checked.
> I assume that for hw implementation, that would be the same.
> 
> But anyway, this discussion would be rather part of the spec/device
> patch, don't you think?
> 
> 
> >
> >Might be good to point to the definition commit:
> >https://github.com/oasis-tcs/virtio-spec/commit/4f1981a1ff46b7aeb801c4c524ff76e93d9ce022
> 
> There were couple of fixes to the spec since then, that's why I didn't
> include it. It is trivial to look it up in the spec.

This might be a good link:

https://docs.oasis-open.org/virtio/virtio/v1.2/cs01/virtio-v1.2-cs01.html#x1-230006x3


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