linux-kernel - RE: [PATCH v7 1/3] Drivers: hv: vmbus: Add vmbus_requestor data structure for VMBus hardening

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Date:   Mon, 14 Sep 2020 17:29:11 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Andrea Parri <parri.andrea@...il.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        Andres Beltran <lkmlabelt@...il.com>,
        Saruhan Karademir <skarade@...rosoft.com>,
        Juan Vazquez <juvazq@...rosoft.com>
Subject: RE: [PATCH v7 1/3] Drivers: hv: vmbus: Add vmbus_requestor data
 structure for VMBus hardening

From: Andrea Parri <parri.andrea@...il.com> Sent: Tuesday, September 8, 2020 12:54 AM
> 
> > > @@ -300,6 +303,22 @@ int hv_ringbuffer_write(struct vmbus_channel *channel,
> > >  						     kv_list[i].iov_len);
> > >  	}
> > >
> > > +	/*
> > > +	 * Allocate the request ID after the data has been copied into the
> > > +	 * ring buffer.  Once this request ID is allocated, the completion
> > > +	 * path could find the data and free it.
> > > +	 */
> > > +
> > > +	if (desc->flags == VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED) {
> > > +		rqst_id = vmbus_next_request_id(&channel->requestor, requestid);
> > > +		if (rqst_id == VMBUS_RQST_ERROR) {
> > > +			pr_err("No request id available\n");
> > > +			return -EAGAIN;
> > > +		}
> > > +	}
> > > +	desc = hv_get_ring_buffer(outring_info) + old_write;
> > > +	desc->trans_id = (rqst_id == VMBUS_NO_RQSTOR) ? requestid : rqst_id;
> > > +
> >
> > This is a nit, but the above would be clearer to me if written like this:
> >
> > 	flags = desc->flags;
> > 	if (flags == VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED) {
> > 		rqst_id = vmbus_next_request_id(&channel->requestor, requestid);
> > 		if (rqst_id == VMBUS_RQST_ERROR) {
> > 			pr_err("No request id available\n");
> > 			return -EAGAIN;
> > 		}
> > 	} else {
> > 		rqst_id = requestid;
> > 	}
> > 	desc = hv_get_ring_buffer(outring_info) + old_write;
> > 	desc->trans_id = rqst_id;
> >
> > The value of the flags field controls what will be used as the value for the
> > rqst_id.  Having another test to see which value will be used as the trans_id
> > somehow feels a bit redundant.  And then rqst_id doesn't have to be initialized.
> 
> Agreed, will apply in the next version.
> 

In an offline conversation, Andrea has pointed out that my proposed changes
don't work.  After a second look, I'll agreed that Andrea's code is the best that
can be done, so my comments can be ignored.

Michael