Re: [PATCH] PCI: Allow drivers to claim exclusive access to config regions

[Jonathan Cameron]

On Mon, 29 Mar 2021 09:46:15 -0700
Dan Williams <dan.j.williams@xxxxxxxxx> wrote:

> On Fri, Mar 26, 2021 at 9:12 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> >
> > [+cc Christoph]
> >
> > On Wed, Mar 24, 2021 at 06:23:54PM -0700, Dan Williams wrote:  
> > > The PCIE Data Object Exchange (DOE) mailbox is a protocol run over
> > > configuration cycles. It assumes one initiator at a time is
> > > reading/writing the data registers. If userspace reads from the response
> > > data payload it may steal data that a kernel driver was expecting to
> > > read. If userspace writes to the request payload it may corrupt the
> > > request a driver was trying to send.  
> >
> > IIUC the problem we're talking about is that userspace config access,
> > e.g., via "lspci" or "setpci" may interfere with kernel usage of DOE.
> > I attached what I think are the relevant bits from the spec.
> >
> > It looks to me like config *reads* should not be a problem: A read of
> > Write Data Mailbox always returns 0 and looks innocuous.  A userspace
> > read of Read Data Mailbox may return a DW of the data object, but it
> > doesn't advance the cursor, so it shouldn't interfere with a kernel
> > read.
> >
> > A write to Write Data Mailbox could obviously corrupt an object being
> > written to the device.  A config write to Read Data Mailbox *does*
> > advance the cursor, so that would definitely interfere with a kernel
> > user.
> >
> > So I think we're really talking about an issue with "setpci" and I
> > don't expect "lspci" to be a problem.  "setpci" is a valuable tool,
> > and the fact that it can hose your system is not really news.  I don't
> > know how hard we should work to protect against that.  
> 
> True, the threat is smaller than I was reading, I apologize for that
> noise. Temporary blocking over kernel DOE cycles seems sufficient for
> now.

Was on vacation. Glad I read the whole thread before replying.  The
key point as Bjorn identified is that reads don't hurt and as noted
setpci can break many things anyway so I think we are safe on this.

There are more 'exciting' questions to be addressed about firmware
vs OS ownership of particular DOE mailboxes but that's a whole
separate bit of fun for the future.

Jonathan