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

[Jiri Pirko <jiri@resnulli.us>]

Tue, Feb 21, 2023 at 05:12:33PM CET, willemdebruijn.kernel@gmail.com wrote:
>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.
>
>tap_get_user performs basic bounds checking on the length passed.

Sure. It trusts the hdr_len, but it sanitizes the input.


> 
>> 
>> >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.
>
>A device cannot blindly trust a hdr_len passed from a driver. We have
>had bugs in the kernel with this before, such as the one fixed in
>commit 57031eb79490 ("packet: round up linear to header len").
>
>> But anyway, this discussion would be rather part of the spec/device
>> patch, don't you think?
>
>I disagree. If it's not much effort to make a commit self-documenting
>that is preferable. And if not, then an explicit reference to an
>authoratitive external reference is preferable over "it is trivial to
>look it up".

Sorry, I don't follow. What exactly do you want me to do?


> 
>> 
>> >
>> >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.