Re: [PATCH v6 0/2] Add p2p via dmabuf to habanalabs

[Oded Gabbay]

On Thu, Sep 23, 2021 at 12:22 PM Oded Gabbay <ogabbay@xxxxxxxxxx> wrote:
>
> On Sat, Sep 18, 2021 at 11:38 AM Oded Gabbay <ogabbay@xxxxxxxxxx> wrote:
> >
> > On Fri, Sep 17, 2021 at 3:30 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
> > >
> > > On Thu, Sep 16, 2021 at 10:10:14AM -0300, Jason Gunthorpe wrote:
> > > > On Thu, Sep 16, 2021 at 02:31:34PM +0200, Daniel Vetter wrote:
> > > > > On Wed, Sep 15, 2021 at 10:45:36AM +0300, Oded Gabbay wrote:
> > > > > > On Tue, Sep 14, 2021 at 7:12 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Tue, Sep 14, 2021 at 04:18:31PM +0200, Daniel Vetter wrote:
> > > > > > > > On Sun, Sep 12, 2021 at 07:53:07PM +0300, Oded Gabbay wrote:
> > > > > > > > > Hi,
> > > > > > > > > Re-sending this patch-set following the release of our user-space TPC
> > > > > > > > > compiler and runtime library.
> > > > > > > > >
> > > > > > > > > I would appreciate a review on this.
> > > > > > > >
> > > > > > > > I think the big open we have is the entire revoke discussions. Having the
> > > > > > > > option to let dma-buf hang around which map to random local memory ranges,
> > > > > > > > without clear ownership link and a way to kill it sounds bad to me.
> > > > > > > >
> > > > > > > > I think there's a few options:
> > > > > > > > - We require revoke support. But I've heard rdma really doesn't like that,
> > > > > > > >   I guess because taking out an MR while holding the dma_resv_lock would
> > > > > > > >   be an inversion, so can't be done. Jason, can you recap what exactly the
> > > > > > > >   hold-up was again that makes this a no-go?
> > > > > > >
> > > > > > > RDMA HW can't do revoke.
> > > > >
> > > > > Like why? I'm assuming when the final open handle or whatever for that MR
> > > > > is closed, you do clean up everything? Or does that MR still stick around
> > > > > forever too?
> > > >
> > > > It is a combination of uAPI and HW specification.
> > > >
> > > > revoke here means you take a MR object and tell it to stop doing DMA
> > > > without causing the MR object to be destructed.
> > > >
> > > > All the drivers can of course destruct the MR, but doing such a
> > > > destruction without explicit synchronization with user space opens
> > > > things up to a serious use-after potential that could be a security
> > > > issue.
> > > >
> > > > When the open handle closes the userspace is synchronized with the
> > > > kernel and we can destruct the HW objects safely.
> > > >
> > > > So, the special HW feature required is 'stop doing DMA but keep the
> > > > object in an error state' which isn't really implemented, and doesn't
> > > > extend very well to other object types beyond simple MRs.
> > >
> > > Yeah revoke without destroying the MR doesn't work, and it sounds like
> > > revoke by destroying the MR just moves the can of worms around to another
> > > place.
> > >
> > > > > 1. User A opens gaudi device, sets up dma-buf export
> > > > >
> > > > > 2. User A registers that with RDMA, or anything else that doesn't support
> > > > > revoke.
> > > > >
> > > > > 3. User A closes gaudi device
> > > > >
> > > > > 4. User B opens gaudi device, assumes that it has full control over the
> > > > > device and uploads some secrets, which happen to end up in the dma-buf
> > > > > region user A set up
> > > >
> > > > I would expect this is blocked so long as the DMABUF exists - eg the
> > > > DMABUF will hold a fget on the FD of #1 until the DMABUF is closed, so
> > > > that #3 can't actually happen.
> > > >
> > > > > It's not mlocked memory, it's mlocked memory and I can exfiltrate
> > > > > it.
> > > >
> > > > That's just bug, don't make buggy drivers :)
> > >
> > > Well yeah, but given that habanalabs hand rolled this I can't just check
> > > for the usual things we have to enforce this in drm. And generally you can
> > > just open chardevs arbitrarily, and multiple users fighting over each
> > > another. The troubles only start when you have private state or memory
> > > allocations of some kind attached to the struct file (instead of the
> > > underlying device), or something else that requires device exclusivity.
> > > There's no standard way to do that.
> > >
> > > Plus in many cases you really want revoke on top (can't get that here
> > > unfortunately it seems), and the attempts to get towards a generic
> > > revoke() just never went anywhere. So again it's all hand-rolled
> > > per-subsystem. *insert lament about us not having done this through a
> > > proper subsystem*
> > >
> > > Anyway it sounds like the code takes care of that.
> > > -Daniel
> >
> > Daniel, Jason,
> > Thanks for reviewing this code.
> >
> > Can I get an R-B / A-B from you for this patch-set ?
> >
> > Thanks,
> > Oded
>
> A kind reminder.
>
> Thanks,
> Oded

Hi,
I know last week was LPC and maybe this got lost in the inbox, so I'm
sending it again to make sure you got my request for R-B / A-B.

Thanks,
Oded