Re: [PATCH v2 2/7] Drivers: hv: vmbus: Avoid double fetch of msgtype in vmbus_on_msg_dpc()

[Wei Liu <wei.liu@kernel.org>]

On Wed, Dec 02, 2020 at 10:22:09AM +0100, Andrea Parri (Microsoft) wrote:
> vmbus_on_msg_dpc() double fetches from msgtype.  The double fetch can
> lead to an out-of-bound access when accessing the channel_message_table
> array.  In turn, the use of the out-of-bound entry could lead to code
> execution primitive (entry->message_handler()).  Avoid the double fetch
> by saving the value of msgtype into a local variable.
> 
> Reported-by: Juan Vazquez <juvazq@microsoft.com>
> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
> ---
>  drivers/hv/vmbus_drv.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 0a2711aa63a15..82b23baa446d7 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1057,6 +1057,7 @@ void vmbus_on_msg_dpc(unsigned long data)
>  	struct hv_message *msg = (struct hv_message *)page_addr +
>  				  VMBUS_MESSAGE_SINT;
>  	struct vmbus_channel_message_header *hdr;
> +	enum vmbus_channel_message_type msgtype;
>  	const struct vmbus_channel_message_table_entry *entry;
>  	struct onmessage_work_context *ctx;
>  	u32 message_type = msg->header.message_type;
> @@ -1072,12 +1073,19 @@ void vmbus_on_msg_dpc(unsigned long data)
>  		/* no msg */
>  		return;
>  
> +	/*
> +	 * The hv_message object is in memory shared with the host.  The host
> +	 * could erroneously or maliciously modify such object.  Make sure to
> +	 * validate its fields and avoid double fetches whenever feasible.
> +	 */
> +
>  	hdr = (struct vmbus_channel_message_header *)msg->u.payload;
> +	msgtype = hdr->msgtype;

Should READ_ONCE be used here?

Wei.