Re: WARNING in usb_composite_setup_continue

Peter Chen
2020-11-12T09:01:18Z

On 20-11-11 14:47:10, Alan Stern wrote:
> On Tue, Nov 10, 2020 at 10:56:50AM -0500, Alan Stern wrote:
> > Felipe:
> >
> > On Mon, Nov 09, 2020 at 02:29:42PM -0500, Kyungtae Kim wrote:
> > > We report a bug (in linux-5.8.13) found by FuzzUSB (a modified version
> > > of syzkaller).
> > >
> > > (corrected analysis)
> > > This bug happens while continuing a delayed setup message in mass
> > > storage gadget.
> > > To be specific, composite_setup() sets FSG_STATE_CONFIG_CHANGE via
> > > fsg_set_alt() (line 1793),
> > > and followed by cdev->delayed_status++ (line 1798).
> > > Meanwile, the mass gadget tries check cdev->delayed_status == 0
> > > through handle_exception() (line 2428),
> > > which occurs in between the two operations above.
> > > Such a race causes invalid operations eventually.
> >
> > Do you know who maintains composite.c (or the composite framework) these
> > days? This is a real race, and it needs to be fixed.
> >
> > Part of the problem seems to be that cdev->delayed_status is sometimes
> > accessed without the protection of cdev->lock. But I don't know when it
> > is safe to take that lock, so I can't tell what changes to make.
> >
> > Another part of the problem is that cdev->delayed_status doesn't count
> > things properly. That is, its value is incremented each time a function
> > driver asks for a delayed status and decremented each time a function
> > driver calls usb_composite_setup_continue(), and the delayed status
> > response is sent when the value reaches 0. But there's nothing to stop
> > this from happening (imagine a gadget with two functions A and B):
> >
> > Function driver A asks for delayed status;
> > Function driver A calls setup_continue(): Now the value
> > of the counter is 0 so a status message is queued
> > too early;
> > Function driver B asks for delayed status;
> > Function driver B calls setup_continue(): Now a second
> > status message is queued.
> >
> > I'm willing to help fix these issues, but I need assistance from someone
> > who fully understands the composite framework.
>
> Okay, so as Peter pointed out, these comments were wrong. The cdev lock
> is held and the scenario described above can't occur, because the lock
> isn't released until after both functions have asked for delayed status.
>
> This means that Kyungtae's analysis of the FuzzUSB report is wrong, and
> we still don't know what really happened. Here's my guess...
>
> There's still a possible race, although it's a different one: The
> gadget's delayed status reply can race with the host timing out and
> sending a new SETUP packet:
>
> Host sends SETUP packet A
>
> Function receives A and decides
> to send a delayed status reply
>
> Function thread starts to
> process packet A
>
> Host times out waiting for A status
>
> Host sends new SETUP packet B
>
> Composite core receives packet B
> and resets cdev->delayed_status

resets cdev->delayed_status? Where to do that? Even the host re-try the
control transfer, it should send the same control request, eg,
SET_CONFIGURATION, and increase cdev->delayed_status. There is a description
for possible host sends next control request before receiving status
packet at USB 2.0 Spec, CH 5.5.5 Control Transfer Data Sequences

If a Setup transaction is received by an endpoint before a previously
initiated control transfer is completed,
the device must abort the current transfer/operation and
handle the new control Setup transaction. A Setup
transaction should not normally be sent before the completion
of a previous control transfer. However, if a
transfer is aborted, for example, due to errors on the bus,
the host can send the next Setup transaction
prematurely from the endpoint’s perspective.

>
> Function thread finishes and calls
> usb_composite_setup_continue()
>
> The composite core sends a status
> reply for packet A, not packet B
>
> Host receives status for A but thinks
> it is the status for B!
>
> Function thread processes packet B
>
> Function thread finishes and calls
> usb_composite_setup_continue()
>
> The composite core sees
> cdev->delayed_status == 0 and WARNs.
>
> At the moment I don't see how to prevent this sort of race from
> happening. We may need to change the API, giving the composite core a
> way to match up calls to usb_composite_setup_continue() with the
> corresponding call to composite_setup(). But even that wouldn't fix
> the entire problem.
>

Hi Alan,

I more think a possible reset or disconnect occurrence between them, and
composite_disconnect is called. Kyungtae, would you please help test below code?
Besides, would you tell me the test environments during FuzzUSB test? A really host or a
host emulated host? Thanks.

diff --git a/drivers/usb/gadget/composite.c b/drivers/usb/gadget/composite.c
index c6d455f2bb92..4a8cc1277206 100644
--- a/drivers/usb/gadget/composite.c
+++ b/drivers/usb/gadget/composite.c
@@ -2454,7 +2454,8 @@ void usb_composite_setup_continue(struct usb_composite_dev *cdev)
spin_lock_irqsave(&cdev->lock, flags);

if (cdev->delayed_status == 0) {
- WARN(cdev, "%s: Unexpected call\n", __func__);
+ WARN(cdev, "%s: Unexpected call, %s\n", __func__,
+ cdev->config ? "connecting state" : "already disconnected");

} else if (--cdev->delayed_status == 0) {
DBG(cdev, "%s: Completing delayed status\n", __func__);
--

Thanks,
Peter Chen