Re: [PATCH] hv_netvsc: Fix netvsc_start_xmit's return type

[Wei Liu]

On Mon, Apr 27, 2020 at 08:30:43PM -0700, Nathan Chancellor wrote:
> netvsc_start_xmit is used as a callback function for the ndo_start_xmit
> function pointer. ndo_start_xmit's return type is netdev_tx_t but
> netvsc_start_xmit's return type is int.
> 
> This causes a failure with Control Flow Integrity (CFI), which requires
> function pointer prototypes and callback function definitions to match
> exactly. When CFI is in enforcing, the kernel panics. When booting a
> CFI kernel with WSL 2, the VM is immediately terminated because of this:
> 
> $ wsl.exe -d ubuntu
> The Windows Subsystem for Linux instance has terminated.
> 
> Avoid this by using the right return type for netvsc_start_xmit.
> 
> Fixes: fceaf24a943d8 ("Staging: hv: add the Hyper-V virtual network driver")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1009

Please consider pulling in the panic log from #1009 to the commit
message. It is much better than the one line message above.

> Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> ---
> 
> Do note that netvsc_xmit still returns int because netvsc_xmit has a
> potential return from netvsc_vf_xmit, which does not return netdev_tx_t
> because of the call to dev_queue_xmit.
> 
> I am not sure if that is an oversight that was introduced by
> commit 0c195567a8f6e ("netvsc: transparent VF management") or if
> everything works properly as it is now.
> 
> My patch is purely concerned with making the definition match the
> prototype so it should be NFC aside from avoiding the CFI panic.
> 
>  drivers/net/hyperv/netvsc_drv.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/hyperv/netvsc_drv.c b/drivers/net/hyperv/netvsc_drv.c
> index d8e86bdbfba1e..ebcfbae056900 100644
> --- a/drivers/net/hyperv/netvsc_drv.c
> +++ b/drivers/net/hyperv/netvsc_drv.c
> @@ -707,7 +707,8 @@ static int netvsc_xmit(struct sk_buff *skb, struct net_device *net, bool xdp_tx)
>  	goto drop;
>  }
>  
> -static int netvsc_start_xmit(struct sk_buff *skb, struct net_device *ndev)
> +static netdev_tx_t netvsc_start_xmit(struct sk_buff *skb,
> +				     struct net_device *ndev)
>  {
>  	return netvsc_xmit(skb, ndev, false);
>  }
> 
> base-commit: 51184ae37e0518fd90cb437a2fbc953ae558cd0d
> -- 
> 2.26.2
>