On 6/30/26 03:01, Christian König wrote:
We recently had another incident where two drivers put pages they got from
get_user_pages() into a DMA-buf and cause quite a number of problems.
Explicitely document that this is not something exporters can do.
I think it would be a good idea to send this up to
https://github.com/masoncl/review-prompts/blob/main/kernel/subsystem/drm.md
as well.
That's what Sashiko uses and it will help flag this in case it happens
in future.
Signed-off-by: Christian König <[email protected]>
---
drivers/dma-buf/dma-buf.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 71f37544a5c6..aa5af4f439c2 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -685,6 +685,14 @@ static struct file *dma_buf_getfile(size_t size, int flags)
*
* For the detailed semantics exporters are expected to implement see
* &dma_buf_ops.
+ *
+ * It is explicitely forbidden for exporters to expose buffers they don't "own"
+ * as DMA-buf. This includes pages acquired by get_user_pages() or other import
+ * mechanism. Not following this rule can create numerous security problems.
+ *
+ * It is also strongly discouraged to expose the same backing store through
+ * multiple DMA-bufs at the same time. This eventually creates aliasing and
+ * cache coherency problems which are extremely hard to debug and fix.
*/
/**
