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

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Date:   Thu, 30 Apr 2020 16:19:22 -0500
From:   Jeremy Linton <jeremy.linton@....com>
To:     linux-usb@...r.kernel.org
Cc:     gregkh@...uxfoundation.org, stern@...land.harvard.edu,
        git@...gavinli.com, jarkko.sakkinen@...ux.intel.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Jeremy Linton <jeremy.linton@....com>
Subject: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

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.

In particular this happens with libusb, when it attempts
to create zero-copy buffers as is used by rtl-sdr, and
maybe other applications. The result is usually
application death.

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@....com>
---
 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)) {
 		dec_usb_memory_use_count(usbm, &usbm->vma_use_count);
 		return -EAGAIN;
 	}
-- 
2.24.1