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

[Greg Kroah-Hartman]

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?

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.

thanks,

greg k-h