dma-buf: Add debug option

We have too many people abusing the struct page they can get at but
really shouldn't in importers. Aside from that the backing page might
simply not exist (for dynamic p2p mappings) looking at it and using it
e.g. for mmap can also wreak the page handling of the exporter
completely. Importers really must go through the proper interface like
dma_buf_mmap for everything.

I'm semi-tempted to enforce this for dynamic importers since those
really have no excuse at all to break the rules.

Unfortuantely we can't store the right pointers somewhere safe to make
sure we oops on something recognizable, so best is to just wrangle
them a bit by flipping all the bits. At least on x86 kernel addresses
have all their high bits sets and the struct page array is fairly low
in the kernel mapping, so flipping all the bits gives us a very high
pointer in userspace and hence excellent chances for an invalid
dereference.

v2: Add a note to the @map_dma_buf hook that exporters shouldn't do
fancy caching tricks, which would blow up with this address scrambling
trick here (Chris)

Enable by default when CONFIG_DMA_API_DEBUG is enabled.

v3: Only one copy of the mangle/unmangle code (Christian)

v4: #ifdef, not #if (0day)

v5: sg_table can also be an ERR_PTR (Chris, Christian)

Reviewed-by: Chris Wilson <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Cc: Chris Wilson <[e-mail]>
Cc: Sumit Semwal <[e-mail]>
Cc: "Christian König" <[e-mail]>
Cc: David Stevens <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/dma-buf/Kconfig

···
 	  This is marked experimental because we don't yet have a consistent
 	  execution context and memory management between drivers.
 
+config DMABUF_DEBUG
+	bool "DMA-BUF debug checks"
+	default y if DMA_API_DEBUG
+	help
+	  This option enables additional checks for DMA-BUF importers and
+	  exporters. Specifically it validates that importers do not peek at the
+	  underlying struct page when they import a buffer.
+
 config DMABUF_SELFTESTS
 	tristate "Selftests for the dma-buf interfaces"
 	default n

drivers/dma-buf/dma-buf.c

···
 }
 EXPORT_SYMBOL_GPL(dma_buf_put);
 
+static void mangle_sg_table(struct sg_table *sg_table)
+{
+#ifdef CONFIG_DMABUF_DEBUG
+	int i;
+	struct scatterlist *sg;
+
+	/* To catch abuse of the underlying struct page by importers mix
+	 * up the bits, but take care to preserve the low SG_ bits to
+	 * not corrupt the sgt. The mixing is undone in __unmap_dma_buf
+	 * before passing the sgt back to the exporter. */
+	for_each_sgtable_sg(sg_table, sg, i)
+		sg->page_link ^= ~0xffUL;
+#endif
+
+}
+static struct sg_table * __map_dma_buf(struct dma_buf_attachment *attach,
+				       enum dma_data_direction direction)
+{
+	struct sg_table *sg_table;
+
+	sg_table = attach->dmabuf->ops->map_dma_buf(attach, direction);
+
+	if (!IS_ERR_OR_NULL(sg_table))
+		mangle_sg_table(sg_table);
+
+	return sg_table;
+}
+
 /**
  * dma_buf_dynamic_attach - Add the device to dma_buf's attachments list
  * @dmabuf:		[in]	buffer to attach device to.
···
 				goto err_unlock;
 		}
 
-		sgt = dmabuf->ops->map_dma_buf(attach, DMA_BIDIRECTIONAL);
+		sgt = __map_dma_buf(attach, DMA_BIDIRECTIONAL);
 		if (!sgt)
 			sgt = ERR_PTR(-ENOMEM);
 		if (IS_ERR(sgt)) {
···
 }
 EXPORT_SYMBOL_GPL(dma_buf_attach);
 
+static void __unmap_dma_buf(struct dma_buf_attachment *attach,
+			    struct sg_table *sg_table,
+			    enum dma_data_direction direction)
+{
+	/* uses XOR, hence this unmangles */
+	mangle_sg_table(sg_table);
+
+	attach->dmabuf->ops->unmap_dma_buf(attach, sg_table, direction);
+}
+
 /**
  * dma_buf_detach - Remove the given attachment from dmabuf's attachments list
  * @dmabuf:	[in]	buffer to detach from.
···
 		if (dma_buf_is_dynamic(attach->dmabuf))
 			dma_resv_lock(attach->dmabuf->resv, NULL);
 
-		dmabuf->ops->unmap_dma_buf(attach, attach->sgt, attach->dir);
+		__unmap_dma_buf(attach, attach->sgt, attach->dir);
 
 		if (dma_buf_is_dynamic(attach->dmabuf)) {
 			dma_buf_unpin(attach);
···
 		}
 	}
 
-	sg_table = attach->dmabuf->ops->map_dma_buf(attach, direction);
+	sg_table = __map_dma_buf(attach, direction);
 	if (!sg_table)
 		sg_table = ERR_PTR(-ENOMEM);
 
···
 	if (dma_buf_is_dynamic(attach->dmabuf))
 		dma_resv_assert_held(attach->dmabuf->resv);
 
-	attach->dmabuf->ops->unmap_dma_buf(attach, sg_table, direction);
+	__unmap_dma_buf(attach, sg_table, direction);
 
 	if (dma_buf_is_dynamic(attach->dmabuf) &&
 	    !IS_ENABLED(CONFIG_DMABUF_MOVE_NOTIFY))

include/linux/dma-buf.h

···
 	 * On failure, returns a negative error value wrapped into a pointer.
 	 * May also return -EINTR when a signal was received while being
 	 * blocked.
+	 *
+	 * Note that exporters should not try to cache the scatter list, or
+	 * return the same one for multiple calls. Caching is done either by the
+	 * DMA-BUF code (for non-dynamic importers) or the importer. Ownership
+	 * of the scatter list is transferred to the caller, and returned by
+	 * @unmap_dma_buf.
 	 */
 	struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
 					 enum dma_data_direction);