Re: [Xen-devel] [PATCH v2 7/9] xen/gntdev: Implement dma-buf export functionality

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

On 06/07/2018 12:48 AM, Boris Ostrovsky wrote:
> On 06/06/2018 08:10 AM, Oleksandr Andrushchenko wrote:
> > On 06/05/2018 01:07 AM, Boris Ostrovsky wrote:
> > > On 06/01/2018 07:41 AM, Oleksandr Andrushchenko wrote:
> > > +
> > > +static struct sg_table *
> > > +dmabuf_exp_ops_map_dma_buf(struct dma_buf_attachment *attach,
> > > +               enum dma_data_direction dir)
> > > +{
> > > +    struct gntdev_dmabuf_attachment *gntdev_dmabuf_attach =
> > > attach->priv;
> > > +    struct gntdev_dmabuf *gntdev_dmabuf = attach->dmabuf->priv;
> > > +    struct sg_table *sgt;
> > > +
> > > +    pr_debug("Mapping %d pages for dev %p\n", gntdev_dmabuf->nr_pages,
> > > +         attach->dev);
> > > +
> > > +    if (WARN_ON(dir == DMA_NONE || !gntdev_dmabuf_attach))
> > >
> > > WARN_ON_ONCE. Here and elsewhere.
> > Why? The UAPI may be used by different applications, thus we might
> > lose warnings for some of them. Having WARN_ON will show problems
> > for multiple users, not for the first one.
> > Does this make sense to still use WARN_ON?
> Just as with pr_err call somewhere else the concern here is that
> userland (which I think is where this is eventually called from?) may
> intentionally trigger the error, flooding the log.
>
> And even this is not directly called from userland there is still a
> possibility of triggering this error multiple times.
Ok, will use WARN_ON_ONCE
> > > > +
> > > > +    if (use_ptemod) {
> > > > +        pr_err("Cannot provide dma-buf: use_ptemode %d\n",
> > > > +               use_ptemod);
> > > No pr_err here please. This can potentially become a DoS vector as it
> > > comes directly from ioctl.
> > >
> > > I would, in fact, revisit other uses of pr_err in this file.
> > Sure, all of pr_err can actually be pr_debug...
> I'd check even further and see if any prink is needed. I think I saw a
> couple that were not especially useful.
All those were useful while debugging the code and use-cases,
so I would prefer to have them all still available, but as pr_debug
instead of pr_err
If hyper_dmabuf will use this Xen dma-buf solution then I believe
those will help as well
> > > > +        return -EINVAL;
> > > > +    }
> > > > +
> > > > +    map = dmabuf_exp_alloc_backing_storage(priv, flags, count);
> > > @count comes from userspace. dmabuf_exp_alloc_backing_storage only
> > > checks for it to be >0. Should it be checked for some sane max value?
> > This is not easy as it is hard to tell what could be that
> > max value. For DMA buffers if count is too big then allocation
> > will fail, so need to check for max here  (dma_alloc_{xxx} will
> > filter out too big allocations).
> OK, that may be sufficient. BTW, I believe there were other loops with
> @count being the control variable. Please see if a user can pass a bogus
> value.
Will check for op.count in IOCTLs
> > For Xen balloon allocations I cannot tell what could be that
> > max value neither. Tough question how to limit.
> I think in balloon there is also a guarantee (of sorts) that something
> prior to a loop will fail.
>
>
> -boris
Thank you,
Oleksandr

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