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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Siddharth Chandrasekaran <sidcha@amazon.de>
Cc:     Siddharth Chandrasekaran <sidcha.dev@gmail.com>,
        Liran Alon <liran@amazon.com>,
        Ioannis Aslanidis <iaslan@amazon.de>,
        linux-hyperv@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Siddharth Chandrasekaran <sidcha@amazon.de>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH] asm-generic/hyperv: Fix struct hv_message_header ordering
In-Reply-To: <20210729133702.11383-1-sidcha@amazon.de>
References: <20210729133702.11383-1-sidcha@amazon.de>
Date:   Thu, 29 Jul 2021 15:52:46 +0200
Message-ID: <87eebh9qhd.fsf@vitty.brq.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Siddharth Chandrasekaran <sidcha@amazon.de> writes:

> According to Hyper-V TLFS Version 6.0b, struct hv_message_header members
> should be defined in the order:
>
> 	message_type, reserved, message_flags, payload_size
>
> but we have it defined in the order:
>
> 	message_type, payload_size, message_flags, reserved
>
> that is, the payload_size and reserved members swapped. 

Indeed,

typedef struct
{
	HV_MESSAGE_TYPE MessageType;
	UINT16 Reserved;
	HV_MESSAGE_FLAGS MessageFlags;
	UINT8 PayloadSize;
	union
	{
		UINT64 OriginationId;
		HV_PARTITION_ID Sender;
		HV_PORT_ID Port;
	};
} HV_MESSAGE_HEADER;

> Due to this mix
> up, we were inadvertently causing two issues:
>
>     - The payload_size field has invalid data; it didn't cause an issue
>       so far because we are delivering only timer messages which has fixed
>       size payloads the guest probably did a sizeof(payload) instead
>       relying on the value of payload_size member.
>
>     - The message_flags was always delivered as 0 to the guest;
>       fortunately, according to section 13.3.1 message_flags is also
>       treated as a reserved field.
>
> Although this is not causing an issue now, it might in future (we are
> adding more message types in our VSM implementation) so fix it to
> reflect the specification.

I'm wondering how this works for Linux-on-Hyper-V as
e.g. vmbus_on_msg_dpc() checks for mininum and maximum payload length:

        payload_size = msg_copy.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;
        }

        entry = &channel_message_table[msgtype];

	if (!entry->message_handler)
		goto msg_handled;

	if (payload_size < entry->min_payload_len) {
                WARN_ONCE(1, "message too short: msgtype=%d len=%d\n", msgtype, payload_size);
                goto msg_handled;
        }

Maybe it's vice versa, the header is correct and the documentation is wrong?

>
> Signed-off-by: Siddharth Chandrasekaran <sidcha@amazon.de>
> ---
>  include/asm-generic/hyperv-tlfs.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/asm-generic/hyperv-tlfs.h b/include/asm-generic/hyperv-tlfs.h
> index 56348a541c50..a5540e9b171f 100644
> --- a/include/asm-generic/hyperv-tlfs.h
> +++ b/include/asm-generic/hyperv-tlfs.h
> @@ -284,9 +284,9 @@ union hv_port_id {
>  /* Define synthetic interrupt controller message header. */
>  struct hv_message_header {
>  	__u32 message_type;
> -	__u8 payload_size;
> -	union hv_message_flags message_flags;
>  	__u8 reserved[2];
> +	union hv_message_flags message_flags;
> +	__u8 payload_size;
>  	union {
>  		__u64 sender;
>  		union hv_port_id port;

-- 
Vitaly