tty: n_gsm: fix sleep-in-atomic-context bug in gsm_control_send

The function gsm_dlci_t1() is a timer handler that runs in an
atomic context, but it calls "kzalloc(..., GFP_KERNEL)" that
may sleep. As a result, the sleep-in-atomic-context bug will
happen. The process is shown below:

gsm_dlci_t1()
 gsm_dlci_open()
  gsm_modem_update()
   gsm_modem_upd_via_msc()
    gsm_control_send()
     kzalloc(sizeof(.., GFP_KERNEL) //may sleep

This patch changes the gfp_t parameter of kzalloc() from GFP_KERNEL to
GFP_ATOMIC in order to mitigate the bug.

Fixes: e1eaea4 ("tty: n_gsm line discipline")
Signed-off-by: Duoming Zhou <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/tty/n_gsm.c

@@ -1711,7 +1711,7 @@ static struct gsm_control *gsm_control_send(struct gsm_mux *gsm,
  unsigned int command, u8 *data, int clen)
 {
  struct gsm_control *ctrl = kzalloc(sizeof(struct gsm_control),
- GFP_KERNEL);
+ GFP_ATOMIC);
  unsigned long flags;
  if (ctrl == NULL)
  return NULL;