linux-kernel - Re: possible deadlock in mon_bin_vma

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Message-ID: <Pine.LNX.4.44L0.1911211118450.1553-100000@iolanthe.rowland.org>
Date:   Thu, 21 Nov 2019 11:20:20 -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 Thu, 21 Nov 2019, Pete Zaitcev wrote:

> 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?

I think you're right.  But one thing concerns me: What happens if the 
same buffer is mapped by more than one process?  Do you allow that?  I 
haven't read the code in enough detail to see.

Alan Stern