linux-kernel - Re: BUG: unable to handle kernel NULL pointer dereference in mem_serial

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Date:   Sat, 14 Dec 2019 08:55:17 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        syzbot <syzbot+f4f1e871965064ae689e@...kaller.appspotmail.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        asierra@...-inc.com, ext-kimmo.rautkoski@...sala.com,
        Jiri Slaby <jslaby@...e.com>,
        kai heng feng <kai.heng.feng@...onical.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-serial <linux-serial@...r.kernel.org>,
        mika.westerberg@...ux.intel.com, o.barta89@...il.com,
        paulburton@...nel.org, sr@...x.de,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        yegorslists@...glemail.com
Subject: Re: BUG: unable to handle kernel NULL pointer dereference in
 mem_serial_out

On Sat, Dec 14, 2019 at 09:48:29AM +0900, Tetsuo Handa wrote:
> On 2019/12/14 1:07, Greg KH wrote:
> > On Fri, Dec 13, 2019 at 11:31:08PM +0900, Tetsuo Handa wrote:
> >> On 2019/12/13 19:00, Dmitry Vyukov wrote:
> >>> Easier said than done. "normal user of the serial port" is not really
> >>> a thing in Linux, right? You either have CAP_SYS_ADMIN or not, that's
> >>> not per-device...
> >>> As far as I remember +Tetsuo proposed a config along the lines of
> >>> "restrict only things that legitimately cause damage under a fuzzer
> >>> workload", e.g. freezing filesystems, disabling console output, etc.
> >>> This may be another candidate. But I can't find where that proposal is
> >>> now.
> >>
> >> That suggestion got no response for two months.
> >>
> >>   https://lkml.kernel.org/r/3e4e2b6b-7828-54ab-cf28-db1a396d7e20@i-love.sakura.ne.jp
> >>
> >> Unless we add such kernel config option to upstream kernels, it will become
> >> a whack-a-mole game.
> > 
> > It will be a whack-a-mole game no matter what.
> > 
> > Yes, /dev/mem/ makes no sense to fuzz.  Neither does other things (like
> > serial port memory addresses.)
> 
> /dev/mem makes sense to fuzz. Ditto for other things.

What?  What are you going to find if you randomly start to write to
/dev/mem?  How are we supposed to "fix" that?

> > You just will have a list of things that you "do not fuzz as these are
> > dangerous".  Nothing new here, any os will have that.
> 
> The list of kernel config options will become too complicated to maintain.
> If we can have one kernel config option, we can avoid maintaining
> the list of kernel config options (which keeps changing over time).

Use the newly added security_locked_down() call, that gives you a great
indication that root can cause problems for those things.

And it's not a config thing, it's a functionality thing within features,
as is explicitly shown by this very thread for the serial port memory
location.

thanks,

greg k-h