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

[Jiri Slaby]

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@xxxxxxxxx/
> > Link: https://lore.kernel.org/all/20221212023530.2498025-1-zengheng4@xxxxxxxxxx/
> > Link: https://lore.kernel.org/all/5a994a13-d1f2-87a8-09e4-a877e65ed166@xxxxxxxxxx/
>
> 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