From: Daniel Starke <daniel.starke@xxxxxxxxxxx> 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@xxxxxxxxxxxxxxx Signed-off-by: Daniel Starke <daniel.starke@xxxxxxxxxxx> Link: https://lore.kernel.org/r/20220414094225.4527-5-daniel.starke@xxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- drivers/tty/n_gsm.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/drivers/tty/n_gsm.c +++ b/drivers/tty/n_gsm.c @@ -818,7 +818,7 @@ static int gsm_dlci_data_output(struct g 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);
