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

[Oded Gabbay]

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