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 9361ebfbb79fd1bc8594a487c01ad52cdaa391ea Mon Sep 17 00:00:00 2001 From: Daniel Starke daniel.starke@siemens.com Date: Wed, 4 May 2022 10:17:33 +0200 Subject: [PATCH] tty: n_gsm: fix invalid gsmtty_write_room() result

gsmtty_write() does not prevent the user to use the full fifo size of 4096 bytes as allocated in gsm_dlci_alloc(). However, gsmtty_write_room() tries to limit the return value by 'TX_SIZE' and returns a negative value if the fifo has more than 'TX_SIZE' bytes stored. This is obviously wrong as 'TX_SIZE' is defined as 512. Define 'TX_SIZE' to the fifo size and use it accordingly for allocation to keep the current behavior. Return the correct remaining size of the fifo in gsmtty_write_room() via kfifo_avail().

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/20220504081733.3494-3-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 bcb714031d69..fd8b86dde525 100644 --- a/drivers/tty/n_gsm.c +++ b/drivers/tty/n_gsm.c @@ -137,6 +137,7 @@ struct gsm_dlci { int retries; /* Uplink tty if active */ struct tty_port port; /* The tty bound to this DLCI if there is one */ +#define TX_SIZE 4096 /* Must be power of 2. */ struct kfifo fifo; /* Queue fifo for the DLCI */ int adaption; /* Adaption layer in use */ int prev_adaption; @@ -1731,7 +1732,7 @@ static struct gsm_dlci *gsm_dlci_alloc(struct gsm_mux *gsm, int addr) return NULL; spin_lock_init(&dlci->lock); mutex_init(&dlci->mutex); - if (kfifo_alloc(&dlci->fifo, 4096, GFP_KERNEL) < 0) { + if (kfifo_alloc(&dlci->fifo, TX_SIZE, GFP_KERNEL) < 0) { kfree(dlci); return NULL; } @@ -2976,8 +2977,6 @@ static struct tty_ldisc_ops tty_ldisc_packet = { * Virtual tty side */

-#define TX_SIZE 512 - /** * gsm_modem_upd_via_data - send modem bits via convergence layer * @dlci: channel @@ -3217,7 +3216,7 @@ static unsigned int gsmtty_write_room(struct tty_struct *tty) struct gsm_dlci *dlci = tty->driver_data; if (dlci->state == DLCI_CLOSED) return 0; - return TX_SIZE - kfifo_len(&dlci->fifo); + return kfifo_avail(&dlci->fifo); }

static unsigned int gsmtty_chars_in_buffer(struct tty_struct *tty)