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Date:   Wed, 4 Oct 2023 13:57:58 +0100
From:   Lee Jones <lee@kernel.org>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org,
        Daniel Starke <daniel.starke@siemens.com>,
        Fedor Pchelkin <pchelkin@ispras.ru>,
        Jiri Slaby <jirislaby@kernel.org>,
        linux-serial@vger.kernel.org,
        syzbot+5f47a8cea6a12b77a876@syzkaller.appspotmail.com
Subject: Re: [PATCH 1/1] tty: n_gsm: Avoid sleeping during .write() whilst
 atomic
Message-ID: <20231004125758.GB83257@google.com>
References: <20231003170020.830242-1-lee@kernel.org>
 <2023100320-immorally-outboard-573a@gregkh>
 <20231003185500.GD8453@google.com>
 <2023100457-entail-freefall-06fd@gregkh>
 <20231004090918.GB9374@google.com>
 <2023100425-unwieldy-reaffirm-2a1b@gregkh>
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On Wed, 04 Oct 2023, Greg Kroah-Hartman wrote:

> On Wed, Oct 04, 2023 at 10:09:18AM +0100, Lee Jones wrote:
> > On Wed, 04 Oct 2023, Greg Kroah-Hartman wrote:
> > 
> > > On Tue, Oct 03, 2023 at 07:55:00PM +0100, Lee Jones wrote:
> > > > On Tue, 03 Oct 2023, Greg Kroah-Hartman wrote:
> > > > 
> > > > > On Tue, Oct 03, 2023 at 06:00:20PM +0100, Lee Jones wrote:
> > > > > > The important part of the call stack being:
> > > > > > 
> > > > > >   gsmld_write()             # Takes a lock and disables IRQs
> > > > > >     con_write()
> > > > > >       console_lock()
> > > > > 
> > > > > Wait, why is the n_gsm line discipline being used for a console?
> > > > > 
> > > > > What hardware/protocol wants this to happen?
> > > > > 
> > > > > gsm I thought was for a very specific type of device, not a console.
> > > > > 
> > > > > As per:
> > > > > 	https://www.kernel.org/doc/html/v5.9/driver-api/serial/n_gsm.html
> > > > > this is a specific modem protocol, why is con_write() being called?
> > > > 
> > > > What it's meant for and what random users can make it do are likely to
> > > > be quite separate questions.  This scenario is user driven and can be
> > > > replicated simply by issuing a few syscalls (open, ioctl, write).
> > > 
> > > I would recommend that any distro/system that does not want to support
> > > this specific hardware protocol, just disable it for now (it's marked as
> > > experimental too), if they don't want to deal with the potential
> > > sleep-while-atomic issue.
> > 
> > n_gsm is available on all the systems I have available.  The mention of
> > 'EXPERIMENTAL' in the module description appears to have zero effect on
> > whether distros choose to make it available or not.  If you're saying
> > that we know this module is BROKEN however, then perhaps we should mark
> > it as such.
> 
> Also, I think this requires root to set this line discipline to the
> console, right?  A normal user can't do that, or am I missing a code
> path here?

I haven't been testing long, but yes, early indications show that root
is required.

> Is there a reproducer somewhere for this issue that runs as a normal
> user?  I couldn't find one in the syzbot listings but I might have been
> not looking deep enough.

https://syzkaller.appspot.com/text?tag=ReproC&x=15578d8fa80000

-- 
Lee Jones [李琼斯]