usb: xhci: fix a possible null-pointer dereference in xhci_setup_device()

There is a possible null-pointer dereference related to the wait-completion synchronization mechanism in the function xhci_setup_device().

Consider the following execution scenario:

in drivers/usb/host/xhci-mem.c:

  xhci_mem_init()       // 2381
    if (!xhci->dcbaa)   // 2431  xhci->dcbaa can be NULL
    xhci_mem_cleanup()  // 2548
      xhci_cleanup_command_queue()        // 1888
in drivers/usb/host/xhci-ring.c
        xhci_complete_del_and_free_cmd()  // 1619
          complete(cmd->completion);      // 1608

The variable xhci->dcbaa is checked by an if statement at Line 2431. If xhci->dcbaa is NULL, xhci_mem_cleanup() will be called at Line 2548, which eventually leads to complete() at Line 1608 that wakes up the wait_for_completion().

Consider the wait_for_completion() in drivers/usb/host/xhci.c
xhci_setup_device()

    wait_for_completion(command->completion);       // 4179
    le64_to_cpu(xhci->dcbaa->dev_context_ptrs...)); // 4237

The variable xhci->dcbaa is dereferenced (without being rechecked) after the wait_for_completion(), which can introduce a possible null-pointer dereference.

To address this issue, a NULL check is added to ensure the variable xhci->dcbaa is not NULL when xhci_dbg_trace() is called.

This potential bug was discovered using an experimental static analysis tool developed by our team. The tool deduces complete() and wait_for_completion() pairs using alias analysis. It then applies data flow analysis to detect null-pointer dereferences across synchronization points.

---
 drivers/usb/host/xhci.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

index 899c0effb5d3..1c7322a4e301 100644
@@ -4229,12 +4229,14 @@ static int xhci_setup_device(struct usb_hcd *hcd, struct usb_device *udev,
 	temp_64 = xhci_read_64(xhci, &xhci->op_regs->dcbaa_ptr);
 	xhci_dbg_trace(xhci, trace_xhci_dbg_address,
 			"Op regs DCBAA ptr = %#016llx", temp_64);
-	xhci_dbg_trace(xhci, trace_xhci_dbg_address,
-		"Slot ID %d dcbaa entry @%p = %#016llx",
-		udev->slot_id,
-		&xhci->dcbaa->dev_context_ptrs[udev->slot_id],
-		(unsigned long long)
-		le64_to_cpu(xhci->dcbaa->dev_context_ptrs[udev->slot_id]));
+	if (xhci->dcbaa) {
+		xhci_dbg_trace(xhci, trace_xhci_dbg_address,
+			"Slot ID %d dcbaa entry @%p = %#016llx",
+			udev->slot_id,
+			&xhci->dcbaa->dev_context_ptrs[udev->slot_id],
+			(unsigned long long)
+			le64_to_cpu(xhci->dcbaa->dev_context_ptrs[udev->slot_id]));
+	}
 	xhci_dbg_trace(xhci, trace_xhci_dbg_address,
 			"Output Context DMA address = %#08llx",
 			(unsigned long long)virt_dev->out_ctx->dma);

2.43.0

Hi,

I think it's a false positive, because xhci_mem_init() is only called on driver initialization and on resume from suspend, immediately after an explicit xhci_mem_cleanup(), which would have woken up any waiting tasks (and likely made them crash), but there shouldn't be any.

By the way, is your analyzer not finding the issue that any call to xhci_mem_cleanup() wakes up everybody waiting on the command queue and then sets a bunch of things (including xhci->dcbaa) to NULL shortly thereafter? This race looks like it shouldn't be harder to detect than the things you are doing already.

Of course, all of that would bring more false positives too. Basically, you discovered that calling a cleanup function while something else is still pending, or having some work already pending while initialization isn't yet complete, may not end well.

That's still just bandaid and not a real fix. With static analysis one must always review the output and ask if the problem is real, what it really means for the code and what to do about it. Simply ignoring the missing pointer is rarely the right solution.

Regards,
Michal

Actually, I think neither the above nor the issue you reported is a problem at all, because xhci_setup_device() checks if command->status is COMP_COMMAND_ABORTED and returns early in this case. At a quick glance, other command handlers do the same.

This status code is always assigned by xhci_cleanup_command_queue().

Regards,
Michal