Re: [PATCH v3] media: uvc: don't do DMA on stack

On Tue, Jun 22, 2021 at 08:07:12AM +0000, David Laight wrote:

From: Mauro Carvalho Chehab

quoted

Sent: 21 June 2021 14:40

As warned by smatch:
	drivers/media/usb/uvc/uvc_v4l2.c:911 uvc_ioctl_g_input() error: doing dma on the stack (&i)
	drivers/media/usb/uvc/uvc_v4l2.c:943 uvc_ioctl_s_input() error: doing dma on the stack (&i)

those two functions call uvc_query_ctrl passing a pointer to
a data at the DMA stack. those are used to send URBs via
usb_control_msg(). Using DMA stack is not supported and should
not work anymore on modern Linux versions.

So, use a kmalloc'ed buffer.

...

quoted

+	buf = kmalloc(1, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
 	ret = uvc_query_ctrl(chain->dev, UVC_GET_CUR, chain->selector->id,
 			     chain->dev->intfnum,  UVC_SU_INPUT_SELECT_CONTROL,
-			     &i, 1);
+			     buf, 1);

Thought...

Is kmalloc(1, GFP_KERNEL) guaranteed to return a pointer into
a cache line that will not be accessed by any other code?
(This is slightly weaker than requiring a cache-line aligned
pointer - but very similar.)

As I understand it, on architectures that do not have cache-coherent 
I/O, kmalloc is guaranteed to return a buffer that is 
cacheline-aligned and whose length is a multiple of the cacheline 
size.

Now, whether that buffer ends up being accessed by any other code 
depends on what your driver does with the pointer it gets from 
kmalloc.  :-)

Alan Stern

Without that guarantee you can't use the returned buffer for
read dma unless the memory accesses are coherent.

	David