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Message-ID: <Y7x19w/sEvQyfi9C@rowland.harvard.edu>
Date: Mon, 9 Jan 2023 15:15:51 -0500
From: Alan Stern <stern@...land.harvard.edu>
To: Oliver Neukum <oneukum@...e.com>
Cc: syzbot <syzbot+712fd0e60dda3ba34642@...kaller.appspotmail.com>,
WeitaoWang-oc@...oxin.com, arnd@...db.de,
gregkh@...uxfoundation.org, khalid.masum.92@...il.com,
kishon@...com, linux-kernel@...r.kernel.org,
linux-usb@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] KASAN: use-after-free Read in __usb_hcd_giveback_urb (2)
Returning to an old discussion...
On Mon, Dec 12, 2022 at 01:29:24PM +0100, Oliver Neukum wrote:
>
>
> On 08.12.22 18:40, Alan Stern wrote:
> > On Thu, Dec 08, 2022 at 03:36:45PM +0100, Oliver Neukum wrote:
> > > On 06.12.22 16:38, Alan Stern wrote:
>
> > It's hard to tell what's really going on. Looking at
> > xpad_stop_output(), you see that it doesn't do anything if xpad->type is
> > XTYPE_UNKNOWN. Is that what happened here?
>
> The output anchor in xpad was used. So I have to answer that in the negative.
> > I can't figure out where the underlying race is. Maybe it's not
> > directly connected with anchors after all.
> >
> > > As far as I can tell the order we decrease use_count is correct. But:
> > >
> > > 6ec4147e7bdbd (Hans de Goede 2013-10-09 17:01:41 +0200 1674) usb_anchor_resume_wakeups(anchor);
> > > 94dfd7edfd5c9 (Ming Lei 2013-07-03 22:53:07 +0800 1675) atomic_dec(&urb->use_count);
> > >
> > > Do we need to guarantee memory ordering here?
> >
> > I don't think we need to do anything more. usb_kill_urb() is careful to
> > wait for completion handlers to finish, and we already have
>
> By checking use_count
>
> > smp_mb__after_atomic() barriers in the appropriate places to ensure
> > proper memory ordering.
>
> Do we? Looking at __usb_hcd_giveback_urb():
>
> usb_unanchor_urb(urb);
>
> This is an implicit memory barrier
>
> if (likely(status == 0))
> usb_led_activity(USB_LED_EVENT_HOST);
>
> /* pass ownership to the completion handler */
> urb->status = status;
> /*
> * This function can be called in task context inside another remote
> * coverage collection section, but kcov doesn't support that kind of
> * recursion yet. Only collect coverage in softirq context for now.
> */
> kcov_remote_start_usb_softirq((u64)urb->dev->bus->busnum);
> urb->complete(urb);
> kcov_remote_stop_softirq();
>
> usb_anchor_resume_wakeups(anchor);
> atomic_dec(&urb->use_count);
> /*
> * Order the write of urb->use_count above before the read
> * of urb->reject below. Pairs with the memory barriers in
> * usb_kill_urb() and usb_poison_urb().
> */
> smp_mb__after_atomic();
>
> That is the latest time use_count can go to zero.
> But what is the earliest time the CPU could reorder setting use_count to zero?
> Try as I might the last certain memory barrier I can find in this function
> is usb_unanchor_urb().
> That means another CPU can complete usb_kill_urb() before usb_anchor_resume_wakeups()
> runs.
>
> usb_anchor_resume_wakeups(anchor);
>
> I think we need a memory barrier here, too.
>
> atomic_dec(&urb->use_count);
Please comment on the proposed patch below.
Alan Stern
Index: usb-devel/drivers/usb/core/hcd.c
===================================================================
--- usb-devel.orig/drivers/usb/core/hcd.c
+++ usb-devel/drivers/usb/core/hcd.c
@@ -1563,13 +1563,19 @@ int usb_hcd_submit_urb (struct urb *urb,
usbmon_urb_submit_error(&hcd->self, urb, status);
urb->hcpriv = NULL;
INIT_LIST_HEAD(&urb->urb_list);
- atomic_dec(&urb->use_count);
/*
- * Order the write of urb->use_count above before the read
- * of urb->reject below. Pairs with the memory barriers in
- * usb_kill_urb() and usb_poison_urb().
+ * urb->use_count acts like a refcount, so decrementing it to
+ * 0 must be ordered after earlier accesses (pairs with the
+ * implicit control dependencies in the wait conditions of
+ * usb_kill_urb() and usb_poison_urb()). Also, the decrement
+ * must be ordered before the read of urb->reject below
+ * (pairs with the memory barriers in those same routines).
+ *
+ * Get the effect of full memory barriers before and after
+ * the decrement by using atomic_dec_return() instead of a
+ * simple atomic_dec().
*/
- smp_mb__after_atomic();
+ atomic_dec_return(&urb->use_count);
atomic_dec(&urb->dev->urbnum);
if (atomic_read(&urb->reject))
@@ -1672,13 +1678,19 @@ static void __usb_hcd_giveback_urb(struc
kcov_remote_stop_softirq();
usb_anchor_resume_wakeups(anchor);
- atomic_dec(&urb->use_count);
/*
- * Order the write of urb->use_count above before the read
- * of urb->reject below. Pairs with the memory barriers in
- * usb_kill_urb() and usb_poison_urb().
+ * urb->use_count acts like a refcount, so decrementing it to
+ * 0 must be ordered after earlier accesses (pairs with the
+ * implicit control dependencies in the wait conditions of
+ * usb_kill_urb() and usb_poison_urb()). Also, the decrement
+ * must be ordered before the read of urb->reject below
+ * (pairs with the memory barriers in those same routines).
+ *
+ * Get the effect of full memory barriers before and after
+ * the decrement by using atomic_dec_return() instead of a
+ * simple atomic_dec().
*/
- smp_mb__after_atomic();
+ atomic_dec_return(&urb->use_count);
if (unlikely(atomic_read(&urb->reject)))
wake_up(&usb_kill_urb_queue);
Index: usb-devel/drivers/usb/core/urb.c
===================================================================
--- usb-devel.orig/drivers/usb/core/urb.c
+++ usb-devel/drivers/usb/core/urb.c
@@ -726,6 +726,10 @@ void usb_kill_urb(struct urb *urb)
usb_hcd_unlink_urb(urb, -ENOENT);
wait_event(usb_kill_urb_queue, atomic_read(&urb->use_count) == 0);
+ /*
+ * The test of urb->use_count creates a control dependency
+ * ordering the wait_event() call against any later writes.
+ */
atomic_dec(&urb->reject);
}
@@ -776,6 +780,10 @@ void usb_poison_urb(struct urb *urb)
usb_hcd_unlink_urb(urb, -ENOENT);
wait_event(usb_kill_urb_queue, atomic_read(&urb->use_count) == 0);
+ /*
+ * The test of urb->use_count creates a control dependency
+ * ordering the wait_event() call against any later writes.
+ */
}
EXPORT_SYMBOL_GPL(usb_poison_urb);
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