Re: [PATCH v4 8/8] vfio: Validate dma-buf revocation semantics

Pranjal Shrivastava
2026-01-21T14:47:39Z

On Wed, Jan 21, 2026 at 04:25:28PM +0200, Leon Romanovsky wrote:
> On Wed, Jan 21, 2026 at 02:22:31PM +0000, Pranjal Shrivastava wrote:
> > On Wed, Jan 21, 2026 at 09:47:12AM -0400, Jason Gunthorpe wrote:
> > > On Wed, Jan 21, 2026 at 02:59:16PM +0200, Leon Romanovsky wrote:
> > > > From: Leon Romanovsky <leonro@nvidia.com>
> > > >
> > > > Use the new dma_buf_attach_revocable() helper to restrict attachments to
> > > > importers that support mapping invalidation.
> > > >
> > > > Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> > > > ---
> > > > drivers/vfio/pci/vfio_pci_dmabuf.c | 3 +++
> > > > 1 file changed, 3 insertions(+)
> > > >
> > > > diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > > > index 5fceefc40e27..85056a5a3faf 100644
> > > > --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> > > > +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > > > @@ -31,6 +31,9 @@ static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
> > > > if (priv->revoked)
> > > > return -ENODEV;
> > > >
> > > > + if (!dma_buf_attach_revocable(attachment))
> > > > + return -EOPNOTSUPP;
> > > > +
> > > > return 0;
> > > > }
> > >
> > > We need to push an urgent -rc fix to implement a pin function here
> > > that always fails. That was missed and it means things like rdma can
> > > import vfio when the intention was to block that. It would be bad for
> > > that uAPI mistake to reach a released kernel.
> > >
> > > It's tricky that NULL pin ops means "I support pin" :|
> > >
> >
> > I've been wondering about this for a while now, I've been sitting on the
> > following:
> >
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index a4d8f2ff94e4..962bce959366 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -1133,6 +1133,8 @@ int dma_buf_pin(struct dma_buf_attachment *attach)
> >
> > if (dmabuf->ops->pin)
> > ret = dmabuf->ops->pin(attach);
> > + else
> > + ret = -EOPNOTSUPP;
> >
> > return ret;
> > }
> >
> > But didn't get a chance to dive in the history yet. I thought there's a
> > good reason we didn't have it? Would it break exisitng dmabuf users?
>
> Probably every importer which called to dma_buf_pin() while connecting
> to existing exporters as many in tree implementation don't have ->pin()
> implemented.

Fair point. I agree with Jason that we cannot leave this open for VFIO
and we can have a pin op that always fails.

But at the same time, I'd like to discuss if we should think about
changing the dmabuf core, NULL op == success feels like relying on a bug
I agree that it means the exporter has no mm, but I believe there should
be some way for the importer to know that.. the importer can still
decide to use the exported dmabuf while being aware.

Thanks,
Praan