Re: [PATCH v4 4/7] dma-buf: heaps: restricted_heap: Add dma_ops

Daniel Vetter
2024-01-12T09:49:14Z

On Fri, Jan 12, 2024 at 10:41:14AM +0100, Daniel Vetter wrote:
> On Fri, Jan 12, 2024 at 05:20:11PM +0800, Yong Wu wrote:
> > Add the dma_ops for this restricted heap. For restricted buffer,
> > cache_ops/mmap are not allowed, thus return EPERM for them.
> >
> > Signed-off-by: Yong Wu <yong.wu@mediatek.com>
> > ---
> > drivers/dma-buf/heaps/restricted_heap.c | 103 ++++++++++++++++++++++++
> > 1 file changed, 103 insertions(+)
> >
> > diff --git a/drivers/dma-buf/heaps/restricted_heap.c b/drivers/dma-buf/heaps/restricted_heap.c
> > index 8c266a0f6192..ec4c63d2112d 100644
> > --- a/drivers/dma-buf/heaps/restricted_heap.c
> > +++ b/drivers/dma-buf/heaps/restricted_heap.c
> > @@ -12,6 +12,10 @@
> >
> > #include "restricted_heap.h"
> >
> > +struct restricted_heap_attachment {
> > + struct sg_table *table;
> > +};
> > +
> > static int
> > restricted_heap_memory_allocate(struct restricted_heap *heap, struct restricted_buffer *buf)
> > {
> > @@ -45,6 +49,104 @@ restricted_heap_memory_free(struct restricted_heap *heap, struct restricted_buff
> > ops->memory_free(heap, buf);
> > }
> >
> > +static int restricted_heap_attach(struct dma_buf *dmabuf, struct dma_buf_attachment *attachment)
> > +{
> > + struct restricted_buffer *restricted_buf = dmabuf->priv;
> > + struct restricted_heap_attachment *a;
> > + struct sg_table *table;
> > + int ret;
> > +
> > + a = kzalloc(sizeof(*a), GFP_KERNEL);
> > + if (!a)
> > + return -ENOMEM;
> > +
> > + table = kzalloc(sizeof(*table), GFP_KERNEL);
> > + if (!table) {
> > + ret = -ENOMEM;
> > + goto err_free_attach;
> > + }
> > +
> > + ret = sg_alloc_table(table, 1, GFP_KERNEL);
> > + if (ret)
> > + goto err_free_sgt;
> > + sg_set_page(table->sgl, NULL, restricted_buf->size, 0);
>
> So this is definitely broken and violating the dma-buf api rules. You
> cannot let attach succed and supply a dummy/invalid sg table.
>
> Two options:
>
> - Reject ->attach for all this buffers with -EBUSY and provide instead a
> private api for these secure buffers, similar to how virtio_dma_buf has
> private virto-specific apis. This interface would need to be
> standardized across all arm TEE users, so that we don't have a
> disastrous proliferation of apis.
>
> - Allow ->attach, but _only_ for drivers/devices which can access the
> secure buffer correctly, and only if you can put the right secure buffer
> address into the sg table directly. If dma to a secure buffer for a
> given struct device * will not work correctly (i.e. without data
> corruption), you _must_ reject the attach attempt with -EBUSY.
>
> The 2nd approach would be my preferred one, if it's technically possible.
>
> Also my understanding is that arm TEE is standardized, so I think we'll at
> least want some acks from other soc people whether this will work for them
> too.
>
> Finally the usual drill:
> - this also needs the driver side support, if there's any changes needed.
> Just the new heap isn't enough.

Ok I quickly scrolled through your drm patches and that confirms that the
current dma-buf interface you're implementing is just completely breaking
the api. And you need to paper over that will all kinds of very icky
special-casing.

So definitely need to rethink the overall design between dma-buf heaps and
drivers here.
-Sima

> - and for drm you need open userspace for this. Doesn't have to be the
> full content protection decode pipeline, the drivers in drm that landed
> secure buffer support thus far enabled it using the
> EGL_EXT_protected_content extension using gl, which side steps all the
> complications around content decryption keys and support
>
> Cheers, Sima
>
> > +
> > + a->table = table;
> > + attachment->priv = a;
> > +
> > + return 0;
> > +
> > +err_free_sgt:
> > + kfree(table);
> > +err_free_attach:
> > + kfree(a);
> > + return ret;
> > +}
> > +
> > +static void restricted_heap_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attachment)
> > +{
> > + struct restricted_heap_attachment *a = attachment->priv;
> > +
> > + sg_free_table(a->table);
> > + kfree(a->table);
> > + kfree(a);
> > +}
> > +
> > +static struct sg_table *
> > +restricted_heap_map_dma_buf(struct dma_buf_attachment *attachment, enum dma_data_direction direct)
> > +{
> > + struct restricted_heap_attachment *a = attachment->priv;
> > + struct sg_table *table = a->table;
> > +
> > + return table;
> > +}
> > +
> > +static void
> > +restricted_heap_unmap_dma_buf(struct dma_buf_attachment *attachment, struct sg_table *table,
> > + enum dma_data_direction direction)
> > +{
> > + struct restricted_heap_attachment *a = attachment->priv;
> > +
> > + WARN_ON(a->table != table);
> > +}
> > +
> > +static int
> > +restricted_heap_dma_buf_begin_cpu_access(struct dma_buf *dmabuf, enum dma_data_direction direction)
> > +{
> > + return -EPERM;
> > +}
> > +
> > +static int
> > +restricted_heap_dma_buf_end_cpu_access(struct dma_buf *dmabuf, enum dma_data_direction direction)
> > +{
> > + return -EPERM;
> > +}
> > +
> > +static int restricted_heap_dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
> > +{
> > + return -EPERM;
> > +}
> > +
> > +static void restricted_heap_free(struct dma_buf *dmabuf)
> > +{
> > + struct restricted_buffer *restricted_buf = dmabuf->priv;
> > + struct restricted_heap *heap = dma_heap_get_drvdata(restricted_buf->heap);
> > +
> > + restricted_heap_memory_free(heap, restricted_buf);
> > + kfree(restricted_buf);
> > +}
> > +
> > +static const struct dma_buf_ops restricted_heap_buf_ops = {
> > + .attach = restricted_heap_attach,
> > + .detach = restricted_heap_detach,
> > + .map_dma_buf = restricted_heap_map_dma_buf,
> > + .unmap_dma_buf = restricted_heap_unmap_dma_buf,
> > + .begin_cpu_access = restricted_heap_dma_buf_begin_cpu_access,
> > + .end_cpu_access = restricted_heap_dma_buf_end_cpu_access,
> > + .mmap = restricted_heap_dma_buf_mmap,
> > + .release = restricted_heap_free,
> > +};
> > +
> > static struct dma_buf *
> > restricted_heap_allocate(struct dma_heap *heap, unsigned long size,
> > unsigned long fd_flags, unsigned long heap_flags)
> > @@ -66,6 +168,7 @@ restricted_heap_allocate(struct dma_heap *heap, unsigned long size,
> > if (ret)
> > goto err_free_buf;
> > exp_info.exp_name = dma_heap_get_name(heap);
> > + exp_info.ops = &restricted_heap_buf_ops;
> > exp_info.size = restricted_buf->size;
> > exp_info.flags = fd_flags;
> > exp_info.priv = restricted_buf;
> > --
> > 2.25.1
> >
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

--
Daniel Vetter
Software Engineer, Intel Corporation