[1/2] tty: n_gsm: fix race condition in status line change on dead connections

  From: Daniel Starke <daniel.starke@siemens.com>

gsm_cleanup_mux() cleans up the gsm by closing all DLCIs, stopping all
timers, removing the virtual tty devices and clearing the data queues.
This procedure, however, may cause subsequent changes of the virtual modem
status lines of a DLCI. More data is being added the outgoing data queue
and the deleted kick timer is restarted to handle this. At this point many
resources have already been removed by the cleanup procedure. Thus, a
kernel panic occurs.

Fix this by proving in gsm_modem_update() that the cleanup procedure has
not been started and the mux is still alive.

Note that writing to a virtual tty is already protected by checks against
the DLCI specific connection state.

Fixes: c568f7086c6e ("tty: n_gsm: fix missing timer to handle stalled links")
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 2 ++
 1 file changed, 2 insertions(+)
  

On Thu, Oct 26, 2023 at 07:58:43AM +0200, D. Starke wrote:
> From: Daniel Starke <daniel.starke@siemens.com>
> 
> gsm_cleanup_mux() cleans up the gsm by closing all DLCIs, stopping all
> timers, removing the virtual tty devices and clearing the data queues.
> This procedure, however, may cause subsequent changes of the virtual modem
> status lines of a DLCI. More data is being added the outgoing data queue
> and the deleted kick timer is restarted to handle this. At this point many
> resources have already been removed by the cleanup procedure. Thus, a
> kernel panic occurs.
> 
> Fix this by proving in gsm_modem_update() that the cleanup procedure has
> not been started and the mux is still alive.
> 
> Note that writing to a virtual tty is already protected by checks against
> the DLCI specific connection state.
> 
> Fixes: c568f7086c6e ("tty: n_gsm: fix missing timer to handle stalled links")
> Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
> ---
>  drivers/tty/n_gsm.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index 1f3aba607cd5..0ee7531c9201 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -4108,6 +4108,8 @@ static int gsm_modem_upd_via_msc(struct gsm_dlci *dlci, u8 brk)
>  
>  static int gsm_modem_update(struct gsm_dlci *dlci, u8 brk)
>  {
> +	if (dlci->gsm->dead)
> +		return -EL2HLT;
>  	if (dlci->adaption == 2) {
>  		/* Send convergence layer type 2 empty data frame. */
>  		gsm_modem_upd_via_data(dlci, brk);
> -- 
> 2.34.1
> 

Hi,

This is the friendly patch-bot of Greg Kroah-Hartman.  You have sent him
a patch that has triggered this response.  He used to manually respond
to these common problems, but in order to save his sanity (he kept
writing the same thing over and over, yet to different people), I was
created.  Hopefully you will not take offence and will fix the problem
in your patch and resubmit it so that it can be accepted into the Linux
kernel tree.

You are receiving this message because of the following common error(s)
as indicated below:

- You have marked a patch with a "Fixes:" tag for a commit that is in an
  older released kernel, yet you do not have a cc: stable line in the
  signed-off-by area at all, which means that the patch will not be
  applied to any older kernel releases.  To properly fix this, please
  follow the documented rules in the
  Documentation/process/stable-kernel-rules.rst file for how to resolve
  this.

If you wish to discuss this problem further, or you have questions about
how to resolve this issue, please feel free to respond to this email and
Greg will reply once he has dug out from the pending patches received
from other developers.

thanks,

greg k-h's patch email bot
  

On Thu, Oct 26, 2023 at 10:27:37AM +0200, Greg KH wrote:
> On Thu, Oct 26, 2023 at 07:58:43AM +0200, D. Starke wrote:
> > From: Daniel Starke <daniel.starke@siemens.com>
> > 
> > gsm_cleanup_mux() cleans up the gsm by closing all DLCIs, stopping all
> > timers, removing the virtual tty devices and clearing the data queues.
> > This procedure, however, may cause subsequent changes of the virtual modem
> > status lines of a DLCI. More data is being added the outgoing data queue
> > and the deleted kick timer is restarted to handle this. At this point many
> > resources have already been removed by the cleanup procedure. Thus, a
> > kernel panic occurs.
> > 
> > Fix this by proving in gsm_modem_update() that the cleanup procedure has
> > not been started and the mux is still alive.
> > 
> > Note that writing to a virtual tty is already protected by checks against
> > the DLCI specific connection state.
> > 
> > Fixes: c568f7086c6e ("tty: n_gsm: fix missing timer to handle stalled links")
> > Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
> > ---
> >  drivers/tty/n_gsm.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> > index 1f3aba607cd5..0ee7531c9201 100644
> > --- a/drivers/tty/n_gsm.c
> > +++ b/drivers/tty/n_gsm.c
> > @@ -4108,6 +4108,8 @@ static int gsm_modem_upd_via_msc(struct gsm_dlci *dlci, u8 brk)
> >  
> >  static int gsm_modem_update(struct gsm_dlci *dlci, u8 brk)
> >  {
> > +	if (dlci->gsm->dead)
> > +		return -EL2HLT;
> >  	if (dlci->adaption == 2) {
> >  		/* Send convergence layer type 2 empty data frame. */
> >  		gsm_modem_upd_via_data(dlci, brk);
> > -- 
> > 2.34.1
> > 
> 
> Hi,
> 
> This is the friendly patch-bot of Greg Kroah-Hartman.  You have sent him
> a patch that has triggered this response.  He used to manually respond
> to these common problems, but in order to save his sanity (he kept
> writing the same thing over and over, yet to different people), I was
> created.  Hopefully you will not take offence and will fix the problem
> in your patch and resubmit it so that it can be accepted into the Linux
> kernel tree.
> 
> You are receiving this message because of the following common error(s)
> as indicated below:
> 
> - You have marked a patch with a "Fixes:" tag for a commit that is in an
>   older released kernel, yet you do not have a cc: stable line in the
>   signed-off-by area at all, which means that the patch will not be
>   applied to any older kernel releases.  To properly fix this, please
>   follow the documented rules in the
>   Documentation/process/stable-kernel-rules.rst file for how to resolve
>   this.
> 
> If you wish to discuss this problem further, or you have questions about
> how to resolve this issue, please feel free to respond to this email and
> Greg will reply once he has dug out from the pending patches received
> from other developers.
> 

Note, I'll take this now, and add the tag by hand, just try to remember
it for the future.

thanks,

greg k-h