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
