The patch below does not apply to the 4.9-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 73029a4d7161f8b6c0934553145ef574d2d0c645 Mon Sep 17 00:00:00 2001 From: Daniel Starke daniel.starke@siemens.com Date: Thu, 14 Apr 2022 02:42:22 -0700 Subject: [PATCH] tty: n_gsm: fix reset fifo race condition

gsmtty_write() and gsm_dlci_data_output() properly guard the fifo access. However, gsm_dlci_close() and gsmtty_flush_buffer() modifies the fifo but do not guard this. Add a guard here to prevent race conditions on parallel writes to the fifo.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline") Cc: stable@vger.kernel.org Signed-off-by: Daniel Starke daniel.starke@siemens.com Link: https://lore.kernel.org/r/20220414094225.4527-17-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 f3fb66be8513..15be4a235783 100644 --- a/drivers/tty/n_gsm.c +++ b/drivers/tty/n_gsm.c @@ -1442,13 +1442,17 @@ static int gsm_control_wait(struct gsm_mux *gsm, struct gsm_control *control)

static void gsm_dlci_close(struct gsm_dlci *dlci) { + unsigned long flags; + del_timer(&dlci->t1); if (debug & 8) pr_debug("DLCI %d goes closed.\n", dlci->addr); dlci->state = DLCI_CLOSED; if (dlci->addr != 0) { tty_port_tty_hangup(&dlci->port, false); + spin_lock_irqsave(&dlci->lock, flags); kfifo_reset(&dlci->fifo); + spin_unlock_irqrestore(&dlci->lock, flags); /* Ensure that gsmtty_open() can return. */ tty_port_set_initialized(&dlci->port, 0); wake_up_interruptible(&dlci->port.open_wait); @@ -3148,13 +3152,17 @@ static unsigned int gsmtty_chars_in_buffer(struct tty_struct *tty) static void gsmtty_flush_buffer(struct tty_struct *tty) { struct gsm_dlci *dlci = tty->driver_data; + unsigned long flags; + if (dlci->state == DLCI_CLOSED) return; /* Caution needed: If we implement reliable transport classes then the data being transmitted can't simply be junked once it has first hit the stack. Until then we can just blow it away */ + spin_lock_irqsave(&dlci->lock, flags); kfifo_reset(&dlci->fifo); + spin_unlock_irqrestore(&dlci->lock, flags); /* Need to unhook this DLCI from the transmit queue logic */ }