[PATCH 1/7] tty: n_gsm: fix encoding of control signal octet bit DV

[PATCH 1/7] tty: n_gsm: fix encoding of control signal octet bit DV

[daniel.starke]

n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
the newer 27.010 here. Chapter 5.4.6.3.7 describes the encoding of the
control signal octet used by the MSC (modem status command). The same
encoding is also used in convergence layer type 2 as described in chapter
5.5.2. Table 7 and 24 both require the DV (data valid) bit to be set 1 for
outgoing control signal octets sent by the DTE (data terminal equipment),
i.e. for the initiator side.
Currently, the DV bit is only set if CD (carrier detect) is on, regardless
of the side.

This patch fixes this behavior by setting the DV bit on the initiator side
unconditionally.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 0b1808e3a912..e199315a158e 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -439,7 +439,7 @@ static u8 gsm_encode_modem(const struct gsm_dlci *dlci)
 		modembits |= MDM_RTR;
 	if (dlci->modem_tx & TIOCM_RI)
 		modembits |= MDM_IC;
-	if (dlci->modem_tx & TIOCM_CD)
+	if (dlci->modem_tx & TIOCM_CD || dlci->gsm->initiator)
 		modembits |= MDM_DV;
 	return modembits;
 }
-- 
2.25.1

[PATCH 2/7] tty: n_gsm: fix encoding of command/response bit

[daniel.starke]

n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
the newer 27.010 here. Chapter 5.2.1.2 describes the encoding of the
C/R (command/response) bit. Table 1 shows that the actual encoding of the
C/R bit is inverted if the associated frame is sent by the responder.

The referenced commit fixed here further broke the internal meaning of this
bit in the outgoing path by always setting the C/R bit regardless of the
frame type.

This patch fixes both by setting the C/R bit always consistently for
command (1) and response (0) frames and inverting it later for the
responder where necessary. The meaning of this bit in the debug output
is being preserved and shows the bit as if it was encoded by the initiator.
This reflects only the frame type rather than the encoded combination of
communication side and frame type.

Fixes: cc0f42122a7e ("tty: n_gsm: Modify CR,PF bit when config requester")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 0b1808e3a912..c012733e0402 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -448,7 +448,7 @@ static u8 gsm_encode_modem(const struct gsm_dlci *dlci)
  *	gsm_print_packet	-	display a frame for debug
  *	@hdr: header to print before decode
  *	@addr: address EA from the frame
- *	@cr: C/R bit from the frame
+ *	@cr: C/R bit seen as initiator
  *	@control: control including PF bit
  *	@data: following data bytes
  *	@dlen: length of data
@@ -548,7 +548,7 @@ static int gsm_stuff_frame(const u8 *input, u8 *output, int len)
  *	gsm_send	-	send a control frame
  *	@gsm: our GSM mux
  *	@addr: address for control frame
- *	@cr: command/response bit
+ *	@cr: command/response bit seen as initiator
  *	@control:  control byte including PF bit
  *
  *	Format up and transmit a control frame. These do not go via the
@@ -563,11 +563,15 @@ static void gsm_send(struct gsm_mux *gsm, int addr, int cr, int control)
 	int len;
 	u8 cbuf[10];
 	u8 ibuf[3];
+	int ocr;
+
+	/* toggle C/R coding if not initiator */
+	ocr = cr ^ (gsm->initiator ? 0 : 1);
 
 	switch (gsm->encoding) {
 	case 0:
 		cbuf[0] = GSM0_SOF;
-		cbuf[1] = (addr << 2) | (cr << 1) | EA;
+		cbuf[1] = (addr << 2) | (ocr << 1) | EA;
 		cbuf[2] = control;
 		cbuf[3] = EA;	/* Length of data = 0 */
 		cbuf[4] = 0xFF - gsm_fcs_add_block(INIT_FCS, cbuf + 1, 3);
@@ -577,7 +581,7 @@ static void gsm_send(struct gsm_mux *gsm, int addr, int cr, int control)
 	case 1:
 	case 2:
 		/* Control frame + packing (but not frame stuffing) in mode 1 */
-		ibuf[0] = (addr << 2) | (cr << 1) | EA;
+		ibuf[0] = (addr << 2) | (ocr << 1) | EA;
 		ibuf[1] = control;
 		ibuf[2] = 0xFF - gsm_fcs_add_block(INIT_FCS, ibuf, 2);
 		/* Stuffing may double the size worst case */
@@ -611,7 +615,7 @@ static void gsm_send(struct gsm_mux *gsm, int addr, int cr, int control)
 
 static inline void gsm_response(struct gsm_mux *gsm, int addr, int control)
 {
-	gsm_send(gsm, addr, 1, control);
+	gsm_send(gsm, addr, 0, control);
 }
 
 /**
@@ -1800,10 +1804,10 @@ static void gsm_queue(struct gsm_mux *gsm)
 		goto invalid;
 
 	cr = gsm->address & 1;		/* C/R bit */
+	cr ^= gsm->initiator ? 0 : 1;	/* Flip so 1 always means command */
 
 	gsm_print_packet("<--", address, cr, gsm->control, gsm->buf, gsm->len);
 
-	cr ^= 1 - gsm->initiator;	/* Flip so 1 always means command */
 	dlci = gsm->dlci[address];
 
 	switch (gsm->control) {
-- 
2.25.1

[PATCH 3/7] tty: n_gsm: fix proper link termination after failed open

[daniel.starke]

Trying to open a DLCI by sending a SABM frame may fail with a timeout.
The link is closed on the initiator side without informing the responder
about this event. The responder assumes the link is open after sending a
UA frame to answer the SABM frame. The link gets stuck in a half open
state.

This patch fixes this by initiating the proper link termination procedure
after link setup timeout instead of silently closing it down.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 0b1808e3a912..52224a3494a0 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -1514,7 +1514,7 @@ static void gsm_dlci_t1(struct timer_list *t)
 			dlci->mode = DLCI_MODE_ADM;
 			gsm_dlci_open(dlci);
 		} else {
-			gsm_dlci_close(dlci);
+			gsm_dlci_begin_close(dlci); /* prevent half open link */
 		}
 
 		break;
-- 
2.25.1

[PATCH 4/7] tty: n_gsm: fix NULL pointer access due to DLCI release

[daniel.starke]

The here fixed commit made the tty hangup asynchronous to avoid a circular
locking warning. I could not reproduce this warning. Furthermore, due to
the asynchronous hangup the function call now gets queued up while the
underlying tty is being freed. Depending on the timing this results in a
NULL pointer access in the global work queue scheduler. To be precise in
process_one_work(). Therefore, the previous commit made the issue worse
which it tried to fix.

This patch fixes this by falling back to the old behavior which uses a
blocking tty hangup call before freeing up the associated tty.

Fixes: 7030082a7415 ("tty: n_gsm: avoid recursive locking with async port hangup")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 0b1808e3a912..e63154ef0b6c 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -1748,7 +1748,12 @@ static void gsm_dlci_release(struct gsm_dlci *dlci)
 		gsm_destroy_network(dlci);
 		mutex_unlock(&dlci->mutex);
 
-		tty_hangup(tty);
+		/* We cannot use tty_hangup() because in tty_kref_put() the tty
+		 * driver assumes that the hangup queue is free and reuses it to
+		 * queue release_one_tty() -> NULL pointer panic in
+		 * process_one_work().
+		 */
+		tty_vhangup(tty);
 
 		tty_port_tty_set(&dlci->port, NULL);
 		tty_kref_put(tty);
-- 
2.25.1

[PATCH 5/7] tty: n_gsm: fix wrong tty control line for flow control

[daniel.starke]

tty flow control is handled via gsmtty_throttle() and gsmtty_unthrottle().
Both functions propagate the outgoing hardware flow control state to the
remote side via MSC (modem status command) frames. The local state is taken
from the RTS (ready to send) flag of the tty. However, RTS gets mapped to
DTR (data terminal ready), which is wrong.
This patch corrects this by mapping RTS to RTS.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 0b1808e3a912..d57fd055b489 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -3234,9 +3234,9 @@ static void gsmtty_throttle(struct tty_struct *tty)
 	if (dlci->state == DLCI_CLOSED)
 		return;
 	if (C_CRTSCTS(tty))
-		dlci->modem_tx &= ~TIOCM_DTR;
+		dlci->modem_tx &= ~TIOCM_RTS;
 	dlci->throttled = true;
-	/* Send an MSC with DTR cleared */
+	/* Send an MSC with RTS cleared */
 	gsmtty_modem_update(dlci, 0);
 }
 
@@ -3246,9 +3246,9 @@ static void gsmtty_unthrottle(struct tty_struct *tty)
 	if (dlci->state == DLCI_CLOSED)
 		return;
 	if (C_CRTSCTS(tty))
-		dlci->modem_tx |= TIOCM_DTR;
+		dlci->modem_tx |= TIOCM_RTS;
 	dlci->throttled = false;
-	/* Send an MSC with DTR set */
+	/* Send an MSC with RTS set */
 	gsmtty_modem_update(dlci, 0);
 }
 
-- 
2.25.1

[PATCH 6/7] tty: n_gsm: fix wrong modem processing in convergence layer type 2

[daniel.starke]

The function gsm_process_modem() exists to handle modem status bits of
incoming frames. This includes incoming MSC (modem status command) frames
and convergence layer type 2 data frames. The function, however, was only
designed to handle MSC frames as it expects the command length. Within
gsm_dlci_data() it is wrongly assumed that this is the same as the data
frame length. This is only true if the data frame contains only 1 byte of
payload.

This patch names the length parameter of gsm_process_modem() in a generic
manner to reflect its association. It also corrects all calls to the
function to handle the variable number of modem status octets correctly in
both cases.

Fixes: 7263287af93d ("tty: n_gsm: Fixed logic to decode break signal from modem status")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 0b1808e3a912..e41508062154 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -1017,25 +1017,25 @@ static void gsm_control_reply(struct gsm_mux *gsm, int cmd, const u8 *data,
  *	@tty: virtual tty bound to the DLCI
  *	@dlci: DLCI to affect
  *	@modem: modem bits (full EA)
- *	@clen: command length
+ *	@slen: number of signal octets
  *
  *	Used when a modem control message or line state inline in adaption
  *	layer 2 is processed. Sort out the local modem state and throttles
  */
 
 static void gsm_process_modem(struct tty_struct *tty, struct gsm_dlci *dlci,
-							u32 modem, int clen)
+							u32 modem, int slen)
 {
 	int  mlines = 0;
 	u8 brk = 0;
 	int fc;
 
-	/* The modem status command can either contain one octet (v.24 signals)
-	   or two octets (v.24 signals + break signals). The length field will
-	   either be 2 or 3 respectively. This is specified in section
-	   5.4.6.3.7 of the  27.010 mux spec. */
+	/* The modem status command can either contain one octet (V.24 signals)
+	 * or two octets (V.24 signals + break signals). This is specified in
+	 * section 5.4.6.3.7 of the 07.10 mux spec.
+	 */
 
-	if (clen == 2)
+	if (slen == 1)
 		modem = modem & 0x7f;
 	else {
 		brk = modem & 0x7f;
@@ -1092,6 +1092,7 @@ static void gsm_control_modem(struct gsm_mux *gsm, const u8 *data, int clen)
 	unsigned int brk = 0;
 	struct gsm_dlci *dlci;
 	int len = clen;
+	int slen;
 	const u8 *dp = data;
 	struct tty_struct *tty;
 
@@ -1111,6 +1112,7 @@ static void gsm_control_modem(struct gsm_mux *gsm, const u8 *data, int clen)
 		return;
 	dlci = gsm->dlci[addr];
 
+	slen = len;
 	while (gsm_read_ea(&modem, *dp++) == 0) {
 		len--;
 		if (len == 0)
@@ -1127,7 +1129,7 @@ static void gsm_control_modem(struct gsm_mux *gsm, const u8 *data, int clen)
 		modem |= (brk & 0x7f);
 	}
 	tty = tty_port_tty_get(&dlci->port);
-	gsm_process_modem(tty, dlci, modem, clen);
+	gsm_process_modem(tty, dlci, modem, slen);
 	if (tty) {
 		tty_wakeup(tty);
 		tty_kref_put(tty);
@@ -1593,6 +1595,7 @@ static void gsm_dlci_data(struct gsm_dlci *dlci, const u8 *data, int clen)
 	struct tty_struct *tty;
 	unsigned int modem = 0;
 	int len = clen;
+	int slen = 0;
 
 	if (debug & 16)
 		pr_debug("%d bytes for tty\n", len);
@@ -1605,12 +1608,14 @@ static void gsm_dlci_data(struct gsm_dlci *dlci, const u8 *data, int clen)
 	case 2:		/* Asynchronous serial with line state in each frame */
 		while (gsm_read_ea(&modem, *data++) == 0) {
 			len--;
+			slen++;
 			if (len == 0)
 				return;
 		}
+		slen++;
 		tty = tty_port_tty_get(port);
 		if (tty) {
-			gsm_process_modem(tty, dlci, modem, clen);
+			gsm_process_modem(tty, dlci, modem, slen);
 			tty_kref_put(tty);
 		}
 		fallthrough;
-- 
2.25.1

[PATCH 7/7] tty: n_gsm: fix deadlock in gsmtty_open()

[daniel.starke]

In the current implementation the user may open a virtual tty which then
could fail to establish the underlying DLCI. The function gsmtty_open()
gets stuck in tty_port_block_til_ready() while waiting for a carrier rise.
This happens if the remote side fails to acknowledge the link establishment
request in time or completely. At some point gsm_dlci_close() is called
to abort the link establishment attempt. The function tries to inform the
associated virtual tty by performing a hangup. But the blocking loop within
tty_port_block_til_ready() is not informed about this event.
The patch proposed here fixes this by resetting the initialization state of
the virtual tty to ensure the loop exits and triggering it to make
tty_port_block_til_ready() return.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 0b1808e3a912..e61a47c349e3 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -1451,6 +1451,9 @@ static void gsm_dlci_close(struct gsm_dlci *dlci)
 	if (dlci->addr != 0) {
 		tty_port_tty_hangup(&dlci->port, false);
 		kfifo_reset(&dlci->fifo);
+		/* Ensure that gsmtty_open() can return. */
+		tty_port_set_initialized(&dlci->port, 0);
+		wake_up_interruptible(&dlci->port.open_wait);
 	} else
 		dlci->gsm->dead = true;
 	/* Unregister gsmtty driver,report gsmtty dev remove uevent for user */
-- 
2.25.1