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From:   Michael Kelley <mikelley@microsoft.com>
To:     "Andrea Parri (Microsoft)" <parri.andrea@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        Juan Vazquez <juvazq@microsoft.com>,
        Saruhan Karademir <skarade@microsoft.com>
Subject: RE: [PATCH 3/6] Drivers: hv: vmbus: Avoid double fetch of
 payload_size in vmbus_on_msg_dpc()
Thread-Topic: [PATCH 3/6] Drivers: hv: vmbus: Avoid double fetch of
 payload_size in vmbus_on_msg_dpc()
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Date:   Sun, 6 Dec 2020 17:14:18 +0000
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 <20201118143649.108465-4-parri.andrea@gmail.com>
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From: Andrea Parri (Microsoft) <parri.andrea@gmail.com> Sent: Wednesday, No=
vember 18, 2020 6:37 AM
>=20
> vmbus_on_msg_dpc() double fetches from payload_size.  The double fetch
> can lead to a buffer overflow when (mem)copying the hv_message object.
> Avoid the double fetch by saving the value of payload_size into a local
> variable.

Similar comment here about providing some brief context in the commit
message on the problem that we are guarding against by removing the
double fetch.

I could see combining this patch with the previous one since the
motivation and pattern of the changes are exactly the same, just for
two different fields.

Michael

>=20
> Reported-by: Juan Vazquez <juvazq@microsoft.com>
> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
> ---
>  drivers/hv/vmbus_drv.c | 17 +++++++----------
>  1 file changed, 7 insertions(+), 10 deletions(-)
>=20
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 82b23baa446d7..0e39f1d6182e9 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1056,6 +1056,7 @@ void vmbus_on_msg_dpc(unsigned long data)
>  	void *page_addr =3D hv_cpu->synic_message_page;
>  	struct hv_message *msg =3D (struct hv_message *)page_addr +
>  				  VMBUS_MESSAGE_SINT;
> +	__u8 payload_size =3D msg->header.payload_size;
>  	struct vmbus_channel_message_header *hdr;
>  	enum vmbus_channel_message_type msgtype;
>  	const struct vmbus_channel_message_table_entry *entry;
> @@ -1089,9 +1090,8 @@ void vmbus_on_msg_dpc(unsigned long data)
>  		goto msg_handled;
>  	}
>=20
> -	if (msg->header.payload_size > HV_MESSAGE_PAYLOAD_BYTE_COUNT) {
> -		WARN_ONCE(1, "payload size is too large (%d)\n",
> -			  msg->header.payload_size);
> +	if (payload_size > HV_MESSAGE_PAYLOAD_BYTE_COUNT) {
> +		WARN_ONCE(1, "payload size is too large (%d)\n", payload_size);
>  		goto msg_handled;
>  	}
>=20
> @@ -1100,21 +1100,18 @@ void vmbus_on_msg_dpc(unsigned long data)
>  	if (!entry->message_handler)
>  		goto msg_handled;
>=20
> -	if (msg->header.payload_size < entry->min_payload_len) {
> -		WARN_ONCE(1, "message too short: msgtype=3D%d len=3D%d\n",
> -			  msgtype, msg->header.payload_size);
> +	if (payload_size < entry->min_payload_len) {
> +		WARN_ONCE(1, "message too short: msgtype=3D%d len=3D%d\n", msgtype,
> payload_size);
>  		goto msg_handled;
>  	}
>=20
>  	if (entry->handler_type	=3D=3D VMHT_BLOCKING) {
> -		ctx =3D kmalloc(sizeof(*ctx) + msg->header.payload_size,
> -			      GFP_ATOMIC);
> +		ctx =3D kmalloc(sizeof(*ctx) + payload_size, GFP_ATOMIC);
>  		if (ctx =3D=3D NULL)
>  			return;
>=20
>  		INIT_WORK(&ctx->work, vmbus_onmessage_work);
> -		memcpy(&ctx->msg, msg, sizeof(msg->header) +
> -		       msg->header.payload_size);
> +		memcpy(&ctx->msg, msg, sizeof(msg->header) + payload_size);
>=20
>  		/*
>  		 * The host can generate a rescind message while we
> --
> 2.25.1