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Date:   Wed, 2 Dec 2020 15:14:33 +0100
From:   Andrea Parri <parri.andrea@gmail.com>
To:     Wei Liu <wei.liu@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org,
        "K . Y . Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Michael Kelley <mikelley@microsoft.com>,
        Juan Vazquez <juvazq@microsoft.com>,
        Saruhan Karademir <skarade@microsoft.com>
Subject: Re: [PATCH v2 2/7] Drivers: hv: vmbus: Avoid double fetch of msgtype
 in vmbus_on_msg_dpc()
Message-ID: <20201202141433.GA24359@andrea>
References: <20201202092214.13520-1-parri.andrea@gmail.com>
 <20201202092214.13520-3-parri.andrea@gmail.com>
 <20201202122254.zjhu3cfcq3zwvmvu@liuwe-devbox-debian-v2>
 <20201202133716.GA22763@andrea>
 <20201202134004.5tgrbyijrhvwwmk2@liuwe-devbox-debian-v2>
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On Wed, Dec 02, 2020 at 01:40:04PM +0000, Wei Liu wrote:
> On Wed, Dec 02, 2020 at 02:37:16PM +0100, Andrea Parri wrote:
> > > > @@ -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?
> > 
> > I think it should.  Thank you for pointing this out.
> 
> Glad I can help.
> 
> The same comment applies to other patches as well, of course.

(As discussed offline/for reference:) I can spot a similar case in
patch #3; however, #4 is supposed to make that access 'non-shared'.

I should probably just squash patches #3 and #4; I'll try to do so
in v3...

Thanks,
  Andrea