linux-kernel - Re: [EXTERNAL] [PATCH 1/5] Drivers: hv: copy from message page only what's needed

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Date:   Thu, 2 Apr 2020 22:46:53 +0800
From:   163 <freedomsky1986@....com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        linux-hyperv@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Wei Liu <wei.liu@...nel.org>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Andrea Parri (Microsoft)" <parri.andrea@...il.com>
Subject: Re: [EXTERNAL] [PATCH 1/5] Drivers: hv: copy from message page only
 what's needed

On 4/1/2020 6:36 PM, Vitaly Kuznetsov wrote:
> Hyper-V Interrupt Message Page (SIMP) has 16 256-byte slots for
> messages. Each message comes with a header (16 bytes) which specifies the
> payload length (up to 240 bytes). vmbus_on_msg_dpc(), however, doesn't
> look at the real message length and copies the whole slot to a temporary
> buffer before passing it to message handlers. This is potentially dangerous
> as hypervisor doesn't have to clean the whole slot when putting a new
> message there and a message handler can get access to some data which
> belongs to a previous message.
> 
> Note, this is not currently a problem because all message handlers are
> in-kernel but eventually we may e.g. get this exported to userspace.
> 
> Note also, that this is not a performance critical path: messages (unlike
> events) represent rare events so it doesn't really matter (from performance
> point of view) if we copy too much.
> 
> Fix the issue by taking into account the real message length. The temporary
> buffer allocated by vmbus_on_msg_dpc() remains fixed size for now.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>   drivers/hv/vmbus_drv.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 029378c27421..2b5572146358 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1043,7 +1043,8 @@ void vmbus_on_msg_dpc(unsigned long data)
>   			return;
>   
>   		INIT_WORK(&ctx->work, vmbus_onmessage_work);
> -		memcpy(&ctx->msg, msg, sizeof(*msg));
> +		memcpy(&ctx->msg, msg, sizeof(msg->header) +
> +		       msg->header.payload_size);
>   

Hi Vitaly:
      I think we still need to check whether the payload_size passed from
Hyper-V is valid or not here to avoid cross-border issue before doing
copying.