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Subject: Re: [PATCH] vdpa/mlx5: fix param validation in mlx5_vdpa_get_config()
To:     Stefano Garzarella <sgarzare@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>
Cc:     virtualization@lists.linux-foundation.org,
        Parav Pandit <parav@nvidia.com>, Eli Cohen <elic@nvidia.com>,
        linux-kernel@vger.kernel.org
References: <20210208161741.104939-1-sgarzare@redhat.com>
 <20210208133312-mutt-send-email-mst@kernel.org>
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 <20210209042530-mutt-send-email-mst@kernel.org>
 <20210210100821.aaye2cgmrpwhhzgn@steredhat>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <5b748533-7091-cba1-50c6-3da2049bc354@redhat.com>
Date:   Thu, 18 Feb 2021 13:47:24 +0800
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On 2021/2/10 下午6:08, Stefano Garzarella wrote:
> On Tue, Feb 09, 2021 at 04:31:23AM -0500, Michael S. Tsirkin wrote:
>> On Tue, Feb 09, 2021 at 11:24:03AM +0800, Jason Wang wrote:
>>>
>>> On 2021/2/9 上午2:38, Michael S. Tsirkin wrote:
>>> > On Mon, Feb 08, 2021 at 05:17:41PM +0100, Stefano Garzarella wrote:
>>> > > It's legal to have 'offset + len' equal to
>>> > > sizeof(struct virtio_net_config), since 'ndev->config' is a
>>> > > 'struct virtio_net_config', so we can safely copy its content under
>>> > > this condition.
>>> > >
>>> > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported 
>>> mlx5 devices")
>>> > > Cc: stable@vger.kernel.org
>>> > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>>> > > ---
>>> > >   drivers/vdpa/mlx5/net/mlx5_vnet.c | 2 +-
>>> > >   1 file changed, 1 insertion(+), 1 deletion(-)
>>> > >
>>> > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c 
>>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> > > index dc88559a8d49..10e9b09932eb 100644
>>> > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> > > @@ -1820,7 +1820,7 @@ static void mlx5_vdpa_get_config(struct 
>>> vdpa_device *vdev, unsigned int offset,
>>> > >       struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>> > >       struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>>> > > -    if (offset + len < sizeof(struct virtio_net_config))
>>> > > +    if (offset + len <= sizeof(struct virtio_net_config))
>>> > >           memcpy(buf, (u8 *)&ndev->config + offset, len);
>>> > >   }
>>> > Actually first I am not sure we need these checks at all.
>>> > vhost_vdpa_config_validate already validates the values, right?
>>>
>>>
>>> I think they're working at different levels. There's no guarantee that
>>> vhost-vdpa is the driver for this vdpa device.
>>
>> In fact, get_config returns void, so userspace can easily get
>> trash if it passes incorrect parameters by mistake, there is
>> no way for userspace to find out whether that is the case :(
>>
>> Any objections to returning the # of bytes copied, or -1
>> on error?
>
> Make sense for me, but are we sure we don't break userspace if we 
> return the number of bytes instead of 0 on success?
>
> I had a quick look at QEMU and it looks like we consider success if 
> the return value is >= 0, but I need to check further.


So I think in the vdpa bus level, we can return #bytes and in vhost uAPI 
level, we can return error if the size is not expected otherwise zero?

Thanks


>
>>
>>>
>>> >
>>> > Second, what will happen when we extend the struct and then
>>> > run new userspace on an old kernel? Looks like it will just
>>> > fail right? So what is the plan?
>>>
>>>
>>> In this case, get_config() should match the spec behaviour. That is 
>>> to say
>>> the size of config space depends on the feature negotiated.
>>>
>>> Thanks
>>
>> Yes but spec says config space can be bigger than specified by features:
>>
>>     Drivers MUST NOT limit structure size and device configuration 
>> space size. Instead, drivers SHOULD only
>>     check that device configuration space is large enough to contain 
>> the fields necessary for device operation.
>>
>
> So IIUC in the driver we should copy as much as we can.
>
> If you agree, I can send an RFC series and we can continue the 
> discussion on it, but I think we should queue this patch for stable 
> branches.
>
> Thanks,
> Stefano
>