tty: n_gsm: fix possible out-of-bounds in gsm0_receive()

jira LE-1907
cve CVE-2024-36016
Rebuild_History Non-Buildable kernel-5.14.0-427.37.1.el9_4
commit-author Daniel Starke <daniel.starke@siemens.com>
commit 47388e8

Assuming the following:
- side A configures the n_gsm in basic option mode
- side B sends the header of a basic option mode frame with data length 1
- side A switches to advanced option mode
- side B sends 2 data bytes which exceeds gsm->len
  Reason: gsm->len is not used in advanced option mode.
- side A switches to basic option mode
- side B keeps sending until gsm0_receive() writes past gsm->buf
  Reason: Neither gsm->state nor gsm->len have been reset after
  reconfiguration.

Fix this by changing gsm->count to gsm->len comparison from equal to less
than. Also add upper limit checks against the constant MAX_MRU in
gsm0_receive() and gsm1_receive() to harden against memory corruption of
gsm->len and gsm->mru.

All other checks remain as we still need to limit the data according to the
user configuration and actual payload size.

	Reported-by: j51569436@gmail.com
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218708
	Tested-by: j51569436@gmail.com
Fixes: e1eaea4 ("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/20240424054842.7741-1-daniel.starke@siemens.com
	Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 47388e8)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: PlaidCat

drivers/tty/n_gsm.c

@@ -2661,7 +2661,10 @@ 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 >= MAX_MRU) {
+			gsm->bad_size++;
+			gsm->state = GSM_SEARCH;
+		} else 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,
@@ -2773,7 +2776,7 @@ static void gsm1_receive(struct gsm_mux *gsm, unsigned char c)
 		gsm->state = GSM_DATA;
 		break;
 	case GSM_DATA:		/* Data */
-		if (gsm->count > gsm->mru) {	/* Allow one for the FCS */
+		if (gsm->count > gsm->mru || gsm->count > MAX_MRU) {	/* Allow one for the FCS */
 			gsm->state = GSM_OVERRUN;
 			gsm->bad_size++;
 		} else