linux-kernel - Re: [PATCH 1/1] tty: n_gsm: Avoid sleeping during .write() whilst atomic

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Date:   Wed, 4 Oct 2023 10:40:43 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Starke, Daniel" <daniel.starke@...mens.com>
Cc:     Lee Jones <lee@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Fedor Pchelkin <pchelkin@...ras.ru>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "syzbot+5f47a8cea6a12b77a876@...kaller.appspotmail.com" 
        <syzbot+5f47a8cea6a12b77a876@...kaller.appspotmail.com>
Subject: Re: [PATCH 1/1] tty: n_gsm: Avoid sleeping during .write() whilst
 atomic

On 04. 10. 23, 8:05, Greg Kroah-Hartman wrote:
> On Wed, Oct 04, 2023 at 05:59:09AM +0000, Starke, Daniel wrote:
>>> Daniel, any thoughts?
>>
>> Our application of this protocol is only with specific modems to enable
>> circuit switched operation (handling calls, selecting/querying networks,
>> etc.) while doing packet switched communication (i.e. IP traffic over PPP).
>> The protocol was developed for such use cases.
>>
>> Regarding the issue itself:
>> There was already an attempt to fix all this by switching from spinlocks to
>> mutexes resulting in ~20% performance loss. However, the patch was reverted
>> as it did not handle the T1 timer leading into sleep during atomic within
>> gsm_dlci_t1() on every mutex lock there.
>> There was also a suggestion to fix this in do_con_write() as
>> tty_operations::write() appears to be documented as "not allowed to sleep".
>> The patch for this was rejected. It did not fix the issue within n_gsm.
>>
>> Link: https://lore.kernel.org/all/20221203215518.8150-1-pchelkin@ispras.ru/
>> Link: https://lore.kernel.org/all/20221212023530.2498025-1-zengheng4@huawei.com/
>> Link: https://lore.kernel.org/all/5a994a13-d1f2-87a8-09e4-a877e65ed166@kernel.org/
> 
> Ok, I thought I remembered this, I'll just drop this patch from my
> review queue and wait for a better solution if it ever comes up as this
> isn't a real issue that people are seeing on actual systems, but just a
> syzbot report.

I remember too and it is not easy.

So without actually looking into the code, can we just somehow disallow 
attaching this line discipline to a console (ban such a TIOCSETD) and be 
done with this issue? IOW disallow root to shoot their foot.

regards,
-- 
js
suse labs