Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

[Robin Murphy]

On 2020-05-01 8:05 am, Greg KH wrote:
> On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote:
> > On arm64, and possibly other architectures, requesting
> > IO coherent memory may return Normal-NC if the underlying
> > hardware isn't coherent. If these pages are then
> > remapped into userspace as Normal, that defeats the
> > purpose of getting Normal-NC, as well as resulting in
> > mappings with differing cache attributes.
>
> What is "Normal-NC"?
>
> > In particular this happens with libusb, when it attempts
> > to create zero-copy buffers as is used by rtl-sdr, and
>
> What is "rtl-sdr"
>
> > maybe other applications. The result is usually
> > application death.
>
> So is this a new problem?  Old problem?  Old problem only showing up on
> future devices?  On current devices?  I need a hint here as to know if
> this is a bugfix or just work to make future devices work properly.
>
> > If dma_mmap_attr() is used instead of remap_pfn_range,
> > the page cache/etc attributes can be matched between the
> > kernel and userspace.
> >
> > Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
> > ---
> >   drivers/usb/core/devio.c | 5 ++---
> >   1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
> > index 6833c918abce..1e7458dd6e5d 100644
> > --- a/drivers/usb/core/devio.c
> > +++ b/drivers/usb/core/devio.c
> > @@ -217,6 +217,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
> >   {
> >   	struct usb_memory *usbm = NULL;
> >   	struct usb_dev_state *ps = file->private_data;
> > +	struct usb_hcd *hcd = bus_to_hcd(ps->dev->bus);
> >   	size_t size = vma->vm_end - vma->vm_start;
> >   	void *mem;
> >   	unsigned long flags;
> > @@ -250,9 +251,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
> >   	usbm->vma_use_count = 1;
> >   	INIT_LIST_HEAD(&usbm->memlist);
> >   
> > -	if (remap_pfn_range(vma, vma->vm_start,
> > -			virt_to_phys(usbm->mem) >> PAGE_SHIFT,
> > -			size, vma->vm_page_prot) < 0) {
> > +	if (dma_mmap_attrs(hcd->self.sysdev, vma, mem, dma_handle, size, 0)) {
>
> Given that this code has not changed since 2016, how has no one noticed
> this issue before?

They have. Here's where the most recent one in my inbox ended, which has 
breadcrumbs to a couple more:

https://lore.kernel.org/linux-arm-kernel/20190808130525.GA1756@xxxxxxxxx/

Note the author ;)

From memory, all the previous attempts wound up getting stuck on the 
subtlety that buffers from hcd_alloc() may or may not actually have come 
from the DMA API. Since then, the localmem_pool rework has probably 
helped a bit, but I'm not sure we've ever really nailed down whether 
kmalloc()ed buffers from PIO-mode controllers (i.e. the !hcd_uses_dma() 
case) can end up down this devio path.

Robin.