linux-kernel - Re: possible deadlock in mon_bin_vma

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Date:   Wed, 20 Nov 2019 13:18:07 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Pete Zaitcev <zaitcev@...hat.com>
cc:     syzbot <syzbot+56f9673bb4cdcbeb0e92@...kaller.appspotmail.com>,
        <arnd@...db.de>, <gregkh@...uxfoundation.org>,
        <jrdr.linux@...il.com>, <keescook@...omium.org>,
        <kstewart@...uxfoundation.org>,
        Kernel development list <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>,
        <syzkaller-bugs@...glegroups.com>, <tglx@...utronix.de>,
        <viro@...iv.linux.org.uk>
Subject: Re: possible deadlock in mon_bin_vma_fault

On Wed, 20 Nov 2019, Pete Zaitcev wrote:

> On Wed, 20 Nov 2019 11:14:05 -0500 (EST)
> Alan Stern <stern@...land.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?
> 
> Actually, please hold on a little, I think to think more about this.
> The deadlock is between mon_bin_read and mon_bin_vma_fault.
> To disallow resizing isn't going to fix _that_.

As I understand it (and my understanding is pretty limited, since I
only started to look seriously at the code one day ago), the reason why
mon_bin_vma_fault acquires fetch_lock is to prevent a resize from
happening while the fault is being handled.  Is there another reason?

If you disallow resizing while the buffer is mapped then 
mon_bin_vma_fault won't need to hold fetch_lock at all.  That would fix 
the deadlock, right?

Alan Stern