linux-kernel - Re: possible deadlock in mon_bin_vma

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Date:   Thu, 21 Nov 2019 08:48:42 -0600
From:   Pete Zaitcev <zaitcev@...hat.com>
To:     Alan Stern <stern@...land.harvard.edu>
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>, zaitcev@...hat.com
Subject: Re: possible deadlock in mon_bin_vma_fault

On Wed, 20 Nov 2019 13:47:00 -0500 (EST)
Alan Stern <stern@...land.harvard.edu> wrote:

> > +               if (rp->mmap_active)
> > +                       return -EBUSY;

> Like that, yes, but the test has to be made while fetch_lock is held.  

Certainly, thanks. I was rushing just to add a postscriptum.

> 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.

I probably should change that comment to "protect the integrity of the
circular buffer, such as b_out".

Anyway... If you are looking at it too, what do you think about not using
any locks in mon_bin_vma_fault() at all? Isn't it valid? I think I tried
to be "safe", but it only uses things that are constants unless we're
opening and closing; a process cannot make page faults unless it has
some thing mapped; and that is only possible if device is open and stays
open. Can you find a hole in this reasoning?

-- Pete