tty: n_gsm: fix wrong signal octet encoding in convergence layer type 2

commit 06d5afd4d640eea67f5623e76cd5fc03359b7f3c upstream.

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 5.5.2 describes that the signal octet in
convergence layer type 2 can be either one or two bytes. The length is
encoded in the EA bit. This is set 1 for the last byte in the sequence.
gsmtty_modem_update() handles this correctly but gsm_dlci_data_output()
fails to set EA to 1. There is no case in which we encode two signal octets
as there is no case in which we send out a break signal.
Therefore, always set the EA bit to 1 for the signal octet to fix this.

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/20220414094225.4527-5-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 1ffb27ca02027c57263564c83f10a78fcf1e3ad6..a2546c7acbae7d0496c7f187096ee716adc1a37f 100644 (file)
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -820,7 +820,7 @@ static int gsm_dlci_data_output(struct gsm_mux *gsm, struct gsm_dlci *dlci)
                        break;
                case 2: /* Unstructed with modem bits.
                Always one byte as we never send inline break data */
-                       *dp++ = gsm_encode_modem(dlci);
+                       *dp++ = (gsm_encode_modem(dlci) << 1) | EA;
                        break;
                }
                WARN_ON(kfifo_out_locked(&dlci->fifo, dp , len, &dlci->lock) != len);