[Qemu-devel] [PATCH 04/14] usb-ohci: DMA writeback bug fixes

[Gerd Hoffmann <kraxel@redhat.com>]

From: Wei Yang <weiyang@linux.vnet.ibm.com>

This patch fixes two bugs in the OHCI device where the device writes
back data to system memory that should be exclusively under the
control of the guest side driver.

In OHCI specification Section 5.2.7, it mentioned "In all cases, Host
Controller Driver is responsible for the insertion and removal of all
Endpoint Descriptors in the various Host Controller Endpoint
Descriptor lists".  In the ohci_frame_boundary(), ohci_put_hcca()
writes the entire hcca back including the interrupt ED lists which
should be under driver control. This violates the specification and
can race with a host driver updating that list at the same time.

In the OHCI Spec Section 4.6, Transfer Descriptor Queue Processing, it
mentioned "Since the TD pointed to by TailP is not accessed by the HC,
the Host Controller Driver can initialize that TD and link at least
one other to it without creating a coherency or synchronization
problem".  While the function ohci_put_ed() writes the entire endpoint
descriptor back including the TailP which should under driver
control. This violate the specification and can race with a host
driver updating the TD list at the same time.

In each case the solution is to make sure we don't write data which is
under driver control.

Cc: Gerd Hoffman <kraxel@redhat.com>

Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 hw/usb/hcd-ohci.c |   17 +++++++++++++++--
 1 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/hw/usb/hcd-ohci.c b/hw/usb/hcd-ohci.c
index 51fa111..1a1cc88 100644
--- a/hw/usb/hcd-ohci.c
+++ b/hw/usb/hcd-ohci.c
@@ -121,6 +121,11 @@ struct ohci_hcca {
     uint16_t frame, pad;
     uint32_t done;
 };
+#define HCCA_WRITEBACK_OFFSET   offsetof(struct ohci_hcca, frame)
+#define HCCA_WRITEBACK_SIZE     8 /* frame, pad, done */
+
+#define ED_WBACK_OFFSET offsetof(struct ohci_ed, head)
+#define ED_WBACK_SIZE   4
 
 static void ohci_bus_stop(OHCIState *ohci);
 static void ohci_async_cancel_device(OHCIState *ohci, USBDevice *dev);
@@ -568,7 +573,13 @@ static inline int ohci_read_hcca(OHCIState *ohci,
 static inline int ohci_put_ed(OHCIState *ohci,
                               uint32_t addr, struct ohci_ed *ed)
 {
-    return put_dwords(ohci, addr, (uint32_t *)ed, sizeof(*ed) >> 2);
+    /* ed->tail is under control of the HCD.
+     * Since just ed->head is changed by HC, just write back this
+     */
+
+    return put_dwords(ohci, addr + ED_WBACK_OFFSET,
+                      (uint32_t *)((char *)ed + ED_WBACK_OFFSET),
+                      ED_WBACK_SIZE >> 2);
 }
 
 static inline int ohci_put_td(OHCIState *ohci,
@@ -587,7 +598,9 @@ static inline int ohci_put_iso_td(OHCIState *ohci,
 static inline int ohci_put_hcca(OHCIState *ohci,
                                 uint32_t addr, struct ohci_hcca *hcca)
 {
-    cpu_physical_memory_write(addr + ohci->localmem_base, hcca, sizeof(*hcca));
+    cpu_physical_memory_write(addr + ohci->localmem_base + HCCA_WRITEBACK_OFFSET,
+                              (char *)hcca + HCCA_WRITEBACK_OFFSET,
+                              HCCA_WRITEBACK_SIZE);
     return 1;
 }
 
-- 
1.7.1