linux-kernel - Re: [RFC PATCH 3/4] Drivers: hv: vmbus: Introduce vmbus_sendpacket

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Date:   Fri, 1 Apr 2022 18:09:22 +0200
From:   Andrea Parri <parri.andrea@...il.com>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
Cc:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Wei Hu <weh@...rosoft.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczynski <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 3/4] Drivers: hv: vmbus: Introduce
 vmbus_sendpacket_getid()

> > @@ -354,6 +354,8 @@ int hv_ringbuffer_write(struct vmbus_channel *channel,
> >  	}
> >  	desc = hv_get_ring_buffer(outring_info) + old_write;
> >  	desc->trans_id = (rqst_id == VMBUS_NO_RQSTOR) ? requestid : rqst_id;
> > +	if (trans_id)
> > +		*trans_id = desc->trans_id;
> 
> This line should *not* read the trans_id out of the ring buffer, since that
> memory is shared with the Hyper-V host and subject to being maliciously
> changed by the host.  Need to set *trans_id only from local variables, and
> somehow ensure the compiler doesn't generate code that reads the value
> from the ring buffer.  Maybe mark the desc->trans_id field as volatile, or cast
> it as such?  Or does WRITE_ONCE() work when setting it?

I'd stick to WRITE_ONCE() (with a comment).

Good catch!

Thanks,
  Andrea