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From: Sam Sun <samsun1006219@gmail.com>
Date: Sat, 13 Apr 2024 13:08:41 +0800
Message-ID: <CAEkJfYNJyyGhR9AAWc0V7o8i6pmS+OB=KSbh6XqVWAAGetS9hA@mail.gmail.com>
Subject: Re: [Linux kernel bug] general protection fault in disable_store
To: Alan Stern <stern@rowland.harvard.edu>
Cc: linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org, 
	Greg KH <gregkh@linuxfoundation.org>, swboyd@chromium.org, ricardo@marliere.net, 
	hkallweit1@gmail.com, heikki.krogerus@linux.intel.com, 
	mathias.nyman@linux.intel.com, royluo@google.com, 
	syzkaller-bugs@googlegroups.com, xrivendell7@gmail.com
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On Sat, Apr 13, 2024 at 2:11=E2=80=AFAM Alan Stern <stern@rowland.harvard.e=
du> wrote:
>
> On Sat, Apr 13, 2024 at 12:26:07AM +0800, Sam Sun wrote:
> > On Fri, Apr 12, 2024 at 10:40=E2=80=AFPM Alan Stern <stern@rowland.harv=
ard.edu> wrote:
> > > I suspect the usb_hub_to_struct_hub() call is racing with the
> > > spinlock-protected region in hub_disconnect() (in hub.c).
> > >
> > > > If there is any other thing I could help, please let me know.
> > >
> > > Try the patch below.  It should eliminate that race, which hopefully
> > > will fix the problem.
>
> > I applied this patch and tried to execute several times, no more
> > kernel core dump in my environment. I think this bug is fixed by the
> > patch. But I do have one more question about it. Since it is a data
> > race bug, it has reproducibility issues originally. How can I confirm
> > if a racy bug is fixed by test? This kind of bug might still have a
> > race window but is harder to trigger. Just curious, not for this
> > patch. I think this patch eliminates the racy window.
>
> If you don't what what is racing, then testing cannot prove that a race
> is eliminated.  However, if you do know where a race occurs then it's
> easy to see how mutual exclusion can prevent the race from happening.
>
> In this case the bug might have had a different cause, something other
> than a race between usb_hub_to_struct_hub() and hub_disconnect().  If
> that's so then testing this patch would not be a definite proof that the
> bug is gone.  But if that race _is_ the cause of the bug then this patch
> will fix it -- you can see that just by reading the code with no need
> for testing.
>
> Besides, the patch is needed in any case because that race certainly
> _can_ occur.  And maybe not only on this pathway.
>

Thanks for explaining! I will check the related code next time.

> May I add your "Reported-and-tested-by:" to the patch?

Sure, thanks for your help!

Best Regards,
Yue