Re: [PATCH v2 2/3] tun: Pad virtio header with zero

[Jason Wang]

On Thu, Jan 9, 2025 at 2:59 PM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
>
> tun used to simply advance iov_iter when it needs to pad virtio header,
> which leaves the garbage in the buffer as is. This is especially
> problematic when tun starts to allow enabling the hash reporting
> feature; even if the feature is enabled, the packet may lack a hash
> value and may contain a hole in the virtio header because the packet
> arrived before the feature gets enabled or does not contain the
> header fields to be hashed. If the hole is not filled with zero, it is
> impossible to tell if the packet lacks a hash value.

I'm not sure I will get here, could we do this in the series of hash reporting?

>
> In theory, a user of tun can fill the buffer with zero before calling
> read() to avoid such a problem, but leaving the garbage in the buffer is
> awkward anyway so fill the buffer in tun.
>
> Signed-off-by: Akihiko Odaki <akihiko.odaki@xxxxxxxxxx>
> ---
>  drivers/net/tun_vnet.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/tun_vnet.c b/drivers/net/tun_vnet.c
> index fe842df9e9ef..ffb2186facd3 100644
> --- a/drivers/net/tun_vnet.c
> +++ b/drivers/net/tun_vnet.c
> @@ -138,7 +138,8 @@ int tun_vnet_hdr_put(int sz, struct iov_iter *iter,
>         if (copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr))
>                 return -EFAULT;
>
> -       iov_iter_advance(iter, sz - sizeof(*hdr));
> +       if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr))
> +               return -EFAULT;
>
>         return 0;

There're various callers of iov_iter_advance(), do we need to fix them all?

Thanks

>  }

>
> --
> 2.47.1
>