[PATCH v3] usb: xhci: Fix use-after-free race in xhci_stop_device()

[YuanZhe Liu]

A KASAN slab-use-after-free read was observed in xhci_stop_device():

  BUG: KASAN: slab-use-after-free in xhci_stop_device.constprop.0.isra.0+0x395/0x430
  drivers/usb/host/xhci-hub.c:479
  Read of size 8 at addr ffff88804e85d2f0

  Freed by task 585700:
   kfree+0xf8/0x390 mm/slub.c:6566
   xhci_free_virt_device+0x3f/0x50 drivers/usb/host/xhci-mem.c:881
   xhci_free_dev+0x286/0x350 drivers/usb/host/xhci.c:4131
   usb_disconnect+0x6b6/0x910 drivers/usb/core/hub.c:2391
   ...
   usbdev_ioctl+0x6f05/0x86d0 drivers/usb/core/devio.c:2826

  Allocated by task 572283:
   xhci_alloc_virt_device+0x171/0xb70 drivers/usb/host/xhci-mem.c:986
   xhci_alloc_dev+0x475/0x840 drivers/usb/host/xhci.c:4271
   usb_alloc_dev+0xe8/0xdc0 drivers/usb/core/usb.c:661
   hub_event+0x1c82/0x4050 drivers/usb/core/hub.c:5953

xhci_stop_device() reads xhci->devs[slot_id] without xhci->lock and
keeps using the pointer across xhci_alloc_command(GFP_NOIO), which may
sleep.  All callers (the port suspend and bus suspend paths in
xhci_hub_control() and xhci_bus_suspend()) drop xhci->lock before
calling it, so a concurrent usb_disconnect() -> xhci_free_dev() can
clear xhci->devs[slot_id] and free virt_dev under xhci->lock in that
window.  The endpoint loop then dereferences the freed virt_dev.

commit 88716a93766b ("usb: xhci: Fix panic if disconnect") added a
NULL check for this case, but that only helps when the device was
already freed before the check; it does not close the window.

Look up xhci->devs[slot_id] only after taking xhci->lock, and bail out
if it is NULL or if the device is already being torn down.  The
tracepoint reloads virt_dev->udev, so it moves under the lock and only
runs while the device is still attached.

xhci_free_dev() clears virt_dev->udev without xhci->lock, and only
later detaches the device from xhci->devs[] in xhci_free_virt_device(),
also under the lock.  Take xhci->lock when clearing ->udev so that
xhci_stop_device() can rely on it while holding the same lock.
xhci_free_virt_device() clears xhci->devs[slot_id] and frees the device
under xhci->lock, so a device found there while holding the lock stays
alive until xhci_stop_device() drops the lock, after which virt_dev is
not used anymore.

Fixes: be88fe4f4dda93e3 ("USB: xHCI: port power management implementation")
Cc: stable@xxxxxxxxxxxxxxx
Co-developed-by: Bin Yu <byu@xxxxxxxxxxxxx>
Signed-off-by: Bin Yu <byu@xxxxxxxxxxxxx>
Co-developed-by: Cheng Wen <wencheng@xxxxxxxxxxxxx>
Signed-off-by: Cheng Wen <wencheng@xxxxxxxxxxxxx>
Signed-off-by: YuanZhe Liu <25031212351@xxxxxxxxxxxxxxxxx>
---
Hi Mathias,

Thanks for the review.  Both points are addressed:

Changes since v2:
 - Moved the xhci->devs[slot_id] lookup inside xhci->lock and dropped
   the separate pointer comparison, so the unlocked read before the
   lock is gone.
 - Dropped the explanatory comment.

I kept virt_dev->udev as the marker for "teardown already started"
instead of adding a disable_slot_pending flag.  It is now written
under xhci->lock, so reading it there is enough to tell
xhci_stop_device() not to queue stop endpoint commands, and it keeps
the change small.

The xhci_disable_and_free_slot() race is left for a separate patch.

 drivers/usb/host/xhci-hub.c | 14 +++++++++-----
 drivers/usb/host/xhci.c     |  3 +++
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/host/xhci-hub.c b/drivers/usb/host/xhci-hub.c
index b0264bd8577a..9a3437248e04 100644
--- a/drivers/usb/host/xhci-hub.c
+++ b/drivers/usb/host/xhci-hub.c
@@ -463,17 +463,21 @@ static int xhci_stop_device(struct xhci_hcd *xhci, int slot_id, int suspend)
 	int i;
 
 	ret = 0;
-	virt_dev = xhci->devs[slot_id];
-	if (!virt_dev)
-		return -ENODEV;
-
-	trace_xhci_stop_device(virt_dev);
 
 	cmd = xhci_alloc_command(xhci, true, GFP_NOIO);
 	if (!cmd)
 		return -ENOMEM;
 
 	spin_lock_irqsave(&xhci->lock, flags);
+	virt_dev = xhci->devs[slot_id];
+	if (!virt_dev || !virt_dev->udev) {
+		spin_unlock_irqrestore(&xhci->lock, flags);
+		xhci_free_command(xhci, cmd);
+		return -ENODEV;
+	}
+
+	trace_xhci_stop_device(virt_dev);
+
 	for (i = LAST_EP_INDEX; i > 0; i--) {
 		if (virt_dev->eps[i].ring && virt_dev->eps[i].ring->dequeue) {
 			struct xhci_ep_ctx *ep_ctx;
diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
index 5c759ad28199..9e375eb48f38 100644
--- a/drivers/usb/host/xhci.c
+++ b/drivers/usb/host/xhci.c
@@ -4128,9 +4128,12 @@ static void xhci_free_dev(struct usb_hcd *hcd, struct usb_device *udev)
 	trace_xhci_free_dev(slot_ctx);
 
 	/* Stop any wayward timer functions (which may grab the lock) */
+	spin_lock_irqsave(&xhci->lock, flags);
 	for (i = 0; i < 31; i++)
 		virt_dev->eps[i].ep_state &= ~EP_STOP_CMD_PENDING;
 	virt_dev->udev = NULL;
+	spin_unlock_irqrestore(&xhci->lock, flags);
+
 	xhci_disable_slot(xhci, udev->slot_id);
 
 	spin_lock_irqsave(&xhci->lock, flags);
-- 
2.34.1