linux-kernel - Re: [PATCH 1/1] Drivers: hv: vmbus: Close timing hole that can corrupt per-cpu page Extend the disabling of preemption to include the hypercall so that another thread can't get the CPU and corrupt the per-cpu page used for hypercall arguments.

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Message-ID: <20170517150015.70e97abb@xeon-e3>
Date:   Wed, 17 May 2017 15:00:15 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     mikelley@...hange.microsoft.com
Cc:     mikelley@...rosoft.com, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
        olaf@...fle.de, apw@...onical.com, vkuznets@...hat.com,
        jasowang@...hat.com, leann.ogasawara@...onical.com,
        marcelo.cerri@...onical.com, sthemmin@...rosoft.com,
        kys@...rosoft.com
Subject: Re: [PATCH 1/1] Drivers: hv: vmbus: Close timing hole that can
 corrupt per-cpu page Extend the disabling of preemption to include the
 hypercall so that another thread can't get the CPU and corrupt the per-cpu
 page used for hypercall arguments.

On Wed, 17 May 2017 14:40:55 -0700
mikelley@...hange.microsoft.com wrote:

> From: Michael Kelley <mikelley@...rosoft.com>
> 
> Signed-off-by: Michael Kelley <mikelley@...rosoft.com>
> ---
>  drivers/hv/hv.c |    7 ++++++-
>  1 files changed, 6 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
> index 12e7bae..47ed538 100644
> --- a/drivers/hv/hv.c
> +++ b/drivers/hv/hv.c
> @@ -82,9 +82,14 @@ int hv_post_message(union hv_connection_id connection_id,
>  	aligned_msg->message_type = message_type;
>  	aligned_msg->payload_size = payload_size;
>  	memcpy((void *)aligned_msg->payload, payload, payload_size);
> -	put_cpu_ptr(hv_cpu);
>  
>  	status = hv_do_hypercall(HVCALL_POST_MESSAGE, aligned_msg, NULL);
> +
> +	/* Preemption must remain disabled until after the hypercall
> +	 * so some other thread can't get scheduled onto this cpu and
> +	 * corrupt the per-cpu post_msg_page
> +	 */
> +	put_cpu_ptr(hv_cpu);
>  
>  	return status & 0xFFFF;
>  }

Since Subject line becomes the git log description, it is best practice to keep it short.
On git commit man page:

       Though not required, it’s a good idea to begin the commit message with
       a single short (less than 50 character) line summarizing the change,
       followed by a blank line and then a more thorough description. 

If you need to provide more description put it in the message body.