tty: n_gsm: fix race condition in status line change on dead connections

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")
Cc: stable <stable@kernel.org>
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20231026055844.3127-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 1f3aba607cd51dcc389d1c9ad723643e476e7d13..0ee7531c92017557c6e529448ddd63caf1a7a9fe 100644 (file)
--- 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);