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SubjectRe: [V2][PATCH] ohci-hcd: Fix race condition caused by ohci_urb_enqueue() and io_watchdog_func()
On Fri, 2 Feb 2018, Haiqing Bai wrote:

> From: Shigeru Yoshida <shigeru.yoshida@windriver.com>
>
> Running io_watchdog_func() while ohci_urb_enqueue() is running can
> cause a race condition where ohci->prev_frame_no is corrupted and the
> watchdog can mis-detect following error:
>
> ohci-platform 664a0800.usb: frame counter not updating; disabled
> ohci-platform 664a0800.usb: HC died; cleaning up
>
> Specifically, following scenario causes a race condition:
>
> 1. ohci_urb_enqueue() calls spin_lock_irqsave(&ohci->lock, flags)
> and enters the critical section
> 2. ohci_urb_enqueue() calls timer_pending(&ohci->io_watchdog) and it
> returns false
> 3. ohci_urb_enqueue() sets ohci->prev_frame_no to a frame number
> read by ohci_frame_no(ohci)
> 4. ohci_urb_enqueue() schedules io_watchdog_func() with mod_timer()
> 5. ohci_urb_enqueue() calls spin_unlock_irqrestore(&ohci->lock,
> flags) and exits the critical section
> 6. Later, ohci_urb_enqueue() is called
> 7. ohci_urb_enqueue() calls spin_lock_irqsave(&ohci->lock, flags)
> and enters the critical section
> 8. The timer scheduled on step 4 expires and io_watchdog_func() runs
> 9. io_watchdog_func() calls spin_lock_irqsave(&ohci->lock, flags)
> and waits on it because ohci_urb_enqueue() is already in the
> critical section on step 7
> 10. ohci_urb_enqueue() calls timer_pending(&ohci->io_watchdog) and it
> returns false
> 11. ohci_urb_enqueue() sets ohci->prev_frame_no to new frame number
> read by ohci_frame_no(ohci) because the frame number proceeded
> between step 3 and 6
> 12. ohci_urb_enqueue() schedules io_watchdog_func() with mod_timer()
> 13. ohci_urb_enqueue() calls spin_unlock_irqrestore(&ohci->lock,
> flags) and exits the critical section, then wake up
> io_watchdog_func() which is waiting on step 9
> 14. io_watchdog_func() enters the critical section
> 15. io_watchdog_func() calls ohci_frame_no(ohci) and set frame_no
> variable to the frame number
> 16. io_watchdog_func() compares frame_no and ohci->prev_frame_no
>
> On step 16, because this calling of io_watchdog_func() is scheduled on
> step 4, the frame number set in ohci->prev_frame_no is expected to the
> number set on step 3. However, ohci->prev_frame_no is overwritten on
> step 11. Because step 16 is executed soon after step 11, the frame
> number might not proceed, so ohci->prev_frame_no must equals to
> frame_no.
>
> To address above scenario, this patch introduces a special sentinel
> value IO_WATCHDOG_OFF and set this value to ohci->prev_frame_no when
> the watchdog is not pending or running. When ohci_urb_enqueue()
> schedules the watchdog (step 4 and 12 above), it compares
> ohci->prev_frame_no to IO_WATCHDOG_OFF so that ohci->prev_frame_no is
> not overwritten while io_watchdog_func() is running.
>
> v2: Instead of adding an extra flag variable, defining IO_WATCHDOG_OFF
> as a special sentinel value for prev_frame_no.
>
> Signed-off-by: Shigeru Yoshida <Shigeru.Yoshida@windriver.com>
> Signed-off-by: Haiqing Bai <Haiqing.Bai@windriver.com>

The "v2" explanation is supposed to go below the "---" tear line, but
that doesn't matter much. In any case,

Acked-by: Alan Stern <stern@rowland.harvard.edu>

You know, this could have been fixed a lot more easily if the core
kernel had a timer_pending_or_executing() routine. I bet there are
other places in the kernel that suffer from the same bug and need the
same fix.

Alan Stern

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