tty: n_gsm: fix frame reception handling

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: e1eaea46 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: NDaniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220414094225.4527-6-daniel.starke@siemens.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -219,7 +219,6 @@ struct gsm_mux {
 	int encoding;
 	u8 control;
 	u8 fcs;
-	u8 received_fcs;
 	u8 *txframe;			/* TX framing buffer */
 
 	/* Method for the receiver side */
@@ -1794,18 +1793,7 @@ 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)
@@ -1993,19 +1981,25 @@ 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);
@@ -2024,11 +2018,24 @@ static void gsm0_receive(struct gsm_mux *gsm, unsigned char c)
 static void gsm1_receive(struct gsm_mux *gsm, unsigned char c)
 {
 	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);