[PATCH v2] usb: usbfs: correct kernel->user page attribute mismatch

[Jeremy Linton]

On some architectures (e.g. arm64) requests for
IO coherent memory may use non-cachable attributes if
the relevant device isn't cache coherent. If these
pages are then remapped into userspace as cacheable,
they may not be coherent with the non-cacheable mappings.

In particular this happens with libusb, when it attempts
to create zero-copy buffers for use by rtl-sdr
(https://github.com/osmocom/rtl-sdr/). On low end arm
devices with non-coherent USB ports, the application will
be unexpectedly killed, while continuing to work fine on
arm machines with coherent USB controllers.

This bug has been discovered/reported a few times over
the last few years. In the case of rtl-sdr a compile time
option to enable/disable zero copy was implemented to
work around it.

Rather than relaying on application specific workarounds,
dma_mmap_coherent() can be used instead of remap_pfn_range().
The page cache/etc attributes will then be correctly set in
userspace to match the kernel mapping.

Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
---
v1->v2:
	Update commit message and change to dma_mmap_coherent()
	from dma_mmap_attr(,,,0) which are the same.

 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..b9db9812d6c5 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_coherent(hcd->self.sysdev, vma, mem, dma_handle, size)) {
 		dec_usb_memory_use_count(usbm, &usbm->vma_use_count);
 		return -EAGAIN;
 	}
-- 
2.24.1