Patch "tty: n_gsm: fix user open not possible at responder until initiator open" has been added to the 5.18-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    tty: n_gsm: fix user open not possible at responder until initiator open

to the 5.18-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     tty-n_gsm-fix-user-open-not-possible-at-responder-un.patch
and it can be found in the queue-5.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit c057ed688e653121f570cc4013a6a2675014898f
Author: Daniel Starke <daniel.starke@xxxxxxxxxxx>
Date:   Fri Jul 1 08:16:44 2022 +0200

    tty: n_gsm: fix user open not possible at responder until initiator open
    
    [ Upstream commit ac77f0077c3265197d378158c85a55eee6d21508 ]
    
    After setting up the control channel on both sides the responder side may
    want to open a virtual tty to listen on until the initiator starts an
    application on a user channel. The current implementation allows the
    open() but no other operation, like termios. These fail with EINVAL.
    The responder sided application has no means to detect an open by the
    initiator sided application this way. And the initiator sided applications
    usually expect the responder sided application to listen on the user
    channel upon open.
    Set the user channel into half-open state on responder side once a user
    application opens the virtual tty to allow IO operations on it.
    Furthermore, keep the user channel constipated until the initiator side
    opens it to give the responder sided application the chance to detect the
    new connection and to avoid data loss if the responder sided application
    starts sending before the user channel is open.
    
    Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
    Signed-off-by: Daniel Starke <daniel.starke@xxxxxxxxxxx>
    Link: https://lore.kernel.org/r/20220701061652.39604-1-daniel.starke@xxxxxxxxxxx
    Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 9f2a8c0e1e33..e15b3b107b5e 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -1510,6 +1510,8 @@ static void gsm_dlci_close(struct gsm_dlci *dlci)
 	if (debug & 8)
 		pr_debug("DLCI %d goes closed.\n", dlci->addr);
 	dlci->state = DLCI_CLOSED;
+	/* Prevent us from sending data before the link is up again */
+	dlci->constipated = true;
 	if (dlci->addr != 0) {
 		tty_port_tty_hangup(&dlci->port, false);
 		spin_lock_irqsave(&dlci->lock, flags);
@@ -1539,6 +1541,7 @@ static void gsm_dlci_open(struct gsm_dlci *dlci)
 	del_timer(&dlci->t1);
 	/* This will let a tty open continue */
 	dlci->state = DLCI_OPEN;
+	dlci->constipated = false;
 	if (debug & 8)
 		pr_debug("DLCI %d goes open.\n", dlci->addr);
 	/* Send current modem state */
@@ -1619,6 +1622,25 @@ static void gsm_dlci_begin_open(struct gsm_dlci *dlci)
 	mod_timer(&dlci->t1, jiffies + gsm->t1 * HZ / 100);
 }
 
+/**
+ *	gsm_dlci_set_opening	-	change state to opening
+ *	@dlci: DLCI to open
+ *
+ *	Change internal state to wait for DLCI open from initiator side.
+ *	We set off timers and responses upon reception of an SABM.
+ */
+static void gsm_dlci_set_opening(struct gsm_dlci *dlci)
+{
+	switch (dlci->state) {
+	case DLCI_CLOSED:
+	case DLCI_CLOSING:
+		dlci->state = DLCI_OPENING;
+		break;
+	default:
+		break;
+	}
+}
+
 /**
  *	gsm_dlci_begin_close	-	start channel open procedure
  *	@dlci: DLCI to open
@@ -1762,10 +1784,13 @@ static struct gsm_dlci *gsm_dlci_alloc(struct gsm_mux *gsm, int addr)
 	dlci->addr = addr;
 	dlci->adaption = gsm->adaption;
 	dlci->state = DLCI_CLOSED;
-	if (addr)
+	if (addr) {
 		dlci->data = gsm_dlci_data;
-	else
+		/* Prevent us from sending data before the link is up */
+		dlci->constipated = true;
+	} else {
 		dlci->data = gsm_dlci_command;
+	}
 	gsm->dlci[addr] = dlci;
 	return dlci;
 }
@@ -3178,6 +3203,8 @@ static int gsmtty_open(struct tty_struct *tty, struct file *filp)
 	/* Start sending off SABM messages */
 	if (gsm->initiator)
 		gsm_dlci_begin_open(dlci);
+	else
+		gsm_dlci_set_opening(dlci);
 	/* And wait for virtual carrier */
 	return tty_port_block_til_ready(port, tty, filp);
 }