Re: WARNING in usb_composite_setup_continue

Alan Stern
2020-11-11T15:57:21Z

On Wed, Nov 11, 2020 at 07:59:34AM +0000, Peter Chen wrote:
> On 20-11-10 10:56:50, 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.
> >
>
> Hi Alan & Kyungtae,
>
> I quite not understand why this occurs, since cdev->delayed_status's
> increment and decrement are both protected by cdev->lock.
>
> cdev->delayed_status's increment:
>
> Place 1:
> case USB_REQ_GET_CONFIGURATION:
> spin_lock(&cdev->lock);
> set_config(cdev, ctrl, w_value);
> f->set_alt;
> cdev->delayed_status++;
>
> spin_unlock(&cdev->lock);
>
> Place 2:
> case USB_REQ_SET_INTERFACE:
> spin_lock(&cdev->lock);
> value = f->set_alt(f, w_index, w_value);
> if (value == USB_GADGET_DELAYED_STATUS) {
> DBG(cdev,
> "%s: interface %d (%s) requested delayed status\n",
> __func__, intf, f->name);
> cdev->delayed_status++;
> DBG(cdev, "delayed_status count %d\n",
> cdev->delayed_status);
> }
> spin_unlock(&cdev->lock);
>
> cdev->delayed_status's decrement:
> function: usb_composite_setup_continue which called by fsg_main_thread
> due to FSG_STATE_CONFIG_CHANGE.
>
> spin_lock_irqsave(&cdev->lock, flags);
>
> if (cdev->delayed_status == 0) {
> WARN(cdev, "%s: Unexpected call\n", __func__);
>
> } else if (--cdev->delayed_status == 0) {
> ...
>
> spin_unlock_irqrestore(&cdev->lock, flags);

You are right. I didn't follow the call changes enough to see that
set_config and reset_config are always called with cdev->lock held.

However, the other problem outlined in my earlier email still remains.
But now I understand what's happening well enough to write a patch.

Alan Stern