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Date:   Fri, 2 Feb 2018 10:25:00 -0500 (EST)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Haiqing Bai <Haiqing.Bai@windriver.com>
cc:     gregkh@linuxfoundation.org, <linux-usb@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <Shigeru.Yoshida@windriver.com>
Subject: Re: [V2][PATCH] ohci-hcd: Fix race condition caused by ohci_urb_enqueue()
 and io_watchdog_func()
In-Reply-To: <1517550699-7879-1-git-send-email-Haiqing.Bai@windriver.com>
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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