[PATCH 5.15 120/230] tty: n_gsm: fix frame reception handling

Greg Kroah-Hartman
2022-07-11T09:51:30Z

From: Daniel Starke <daniel.starke@siemens.com>

[ Upstream commit 7a0e4b1733b635026a87c023f6d703faf0095e39 ]

The frame checksum (FCS) is currently handled in gsm_queue() after
reception of a frame. However, this breaks layering. A workaround with
'received_fcs' was implemented so far.
Furthermore, frames are handled as such even if no end flag was received.
Move FCS calculation from gsm_queue() to gsm0_receive() and gsm1_receive().
Also delay gsm_queue() call there until a full frame was received to fix
both points.

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/20220414094225.4527-6-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/tty/n_gsm.c | 53 +++++++++++++++++++++++++--------------------
1 file changed, 30 insertions(+), 23 deletions(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 3038e5631be5..d3d5308daf35 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -221,7 +221,6 @@ struct gsm_mux {
int encoding;
u8 control;
u8 fcs;
- u8 received_fcs;
u8 *txframe; /* TX framing buffer */

/* Method for the receiver side */
@@ -1799,18 +1798,7 @@ static void gsm_queue(struct gsm_mux *gsm)
u8 cr;
int address;
int i, j, k, address_tmp;
- /* We have to sneak a look at the packet body to do the FCS.
- A somewhat layering violation in the spec */

- if ((gsm->control & ~PF) == UI)
- gsm->fcs = gsm_fcs_add_block(gsm->fcs, gsm->buf, gsm->len);
- if (gsm->encoding == 0) {
- /* WARNING: gsm->received_fcs is used for
- gsm->encoding = 0 only.
- In this case it contain the last piece of data
- required to generate final CRC */
- gsm->fcs = gsm_fcs_add(gsm->fcs, gsm->received_fcs);
- }
if (gsm->fcs != GOOD_FCS) {
gsm->bad_fcs++;
if (debug & 4)
@@ -1997,19 +1985,25 @@ static void gsm0_receive(struct gsm_mux *gsm, unsigned char c)
break;
case GSM_DATA: /* Data */
gsm->buf[gsm->count++] = c;
- if (gsm->count == gsm->len)
+ if (gsm->count == gsm->len) {
+ /* Calculate final FCS for UI frames over all data */
+ if ((gsm->control & ~PF) != UIH) {
+ gsm->fcs = gsm_fcs_add_block(gsm->fcs, gsm->buf,
+ gsm->count);
+ }
gsm->state = GSM_FCS;
+ }
break;
case GSM_FCS: /* FCS follows the packet */
- gsm->received_fcs = c;
- gsm_queue(gsm);
+ gsm->fcs = gsm_fcs_add(gsm->fcs, c);
gsm->state = GSM_SSOF;
break;
case GSM_SSOF:
- if (c == GSM0_SOF) {
- gsm->state = GSM_SEARCH;
- break;
- }
+ gsm->state = GSM_SEARCH;
+ if (c == GSM0_SOF)
+ gsm_queue(gsm);
+ else
+ gsm->bad_size++;
break;
default:
pr_debug("%s: unhandled state: %d\n", __func__, gsm->state);
@@ -2038,11 +2032,24 @@ static void gsm1_receive(struct gsm_mux *gsm, unsigned char c)
return;
}
if (c == GSM1_SOF) {
- /* EOF is only valid in frame if we have got to the data state
- and received at least one byte (the FCS) */
- if (gsm->state == GSM_DATA && gsm->count) {
- /* Extract the FCS */
+ /* EOF is only valid in frame if we have got to the data state */
+ if (gsm->state == GSM_DATA) {
+ if (gsm->count < 1) {
+ /* Missing FSC */
+ gsm->malformed++;
+ gsm->state = GSM_START;
+ return;
+ }
+ /* Remove the FCS from data */
gsm->count--;
+ if ((gsm->control & ~PF) != UIH) {
+ /* Calculate final FCS for UI frames over all
+ * data but FCS
+ */
+ gsm->fcs = gsm_fcs_add_block(gsm->fcs, gsm->buf,
+ gsm->count);
+ }
+ /* Add the FCS itself to test against GOOD_FCS */
gsm->fcs = gsm_fcs_add(gsm->fcs, gsm->buf[gsm->count]);
gsm->len = gsm->count;
gsm_queue(gsm);
--
2.35.1