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Date:   Wed, 20 Nov 2019 13:47:00 -0500 (EST)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Pete Zaitcev <zaitcev@redhat.com>
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Subject: Re: possible deadlock in mon_bin_vma_fault
In-Reply-To: <20191120111235.7d306f23@suzdal.zaitcev.lan>
Message-ID: <Pine.LNX.4.44L0.1911201343580.1498-100000@iolanthe.rowland.org>
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On Wed, 20 Nov 2019, Pete Zaitcev wrote:

> On Wed, 20 Nov 2019 11:14:05 -0500 (EST)
> Alan Stern <stern@rowland.harvard.edu> wrote:
> 
> > As it happens, I spent a little time investigating this bug report just
> > yesterday.  It seems to me that the easiest fix would be to disallow
> > resizing the buffer while it is mapped by any users.  (Besides,
> > allowing that seems like a bad idea in any case.)
> > 
> > Pete, does that seem reasonable to you?
> 
> Yes, it does seem reasonable.
> 
> I think I understand it now. My fallacy was thinking that since everything
> is nailed down as long as fetch_lock is held, it was okay to grab whatever
> page from our pagemap. What happens later is an attempt to get pages of the
> new buffer while looking at them through the old VMA, in mon_bin_vma_fault.
> 
> It seems to me that the use counter, mmap_active, is correct and sufficient
> to check in the ioctl.
> 
> -- Pete
> 
> P.S. One thing that vaguely bothers me on this is that the bot
> bisected to the commit that clearly fixed worse issues.
> 
> P.P.S. Like this?
> 
> diff --git a/drivers/usb/mon/mon_bin.c b/drivers/usb/mon/mon_bin.c
> index ac2b4fcc265f..e27d99606adb 100644
> --- a/drivers/usb/mon/mon_bin.c
> +++ b/drivers/usb/mon/mon_bin.c
> @@ -1020,6 +1020,9 @@ static long mon_bin_ioctl(struct file *file, unsigned int cmd, unsigned long arg
>                 int size;
>                 struct mon_pgmap *vec;
>  
> +               if (rp->mmap_active)
> +                       return -EBUSY;
> +
>                 if (arg < BUFF_MIN || arg > BUFF_MAX)
>                         return -EINVAL;

Like that, yes, but the test has to be made while fetch_lock is held.  
Otherwise there's still a race: One thread could pass the test and then
do the resize, and in between another thread could map the buffer and
incur a fault.

Incidentally, the comment for fetch_lock says that it protects b_read 
and b_out, but mon_bin_vma_fault doesn't use either of those fields.

Alan Stern