[PATCH 5.4 002/202] tty: n_gsm: fix possible out-of-bounds in gsm0_receive()

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5.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Daniel Starke <daniel.starke@xxxxxxxxxxx>

commit 47388e807f85948eefc403a8a5fdc5b406a65d5a upstream.

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@xxxxxxxxx
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218708
Tested-by: j51569436@xxxxxxxxx
Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Daniel Starke <daniel.starke@xxxxxxxxxxx>
Link: https://lore.kernel.org/r/20240424054842.7741-1-daniel.starke@xxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/tty/n_gsm.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -1974,8 +1974,12 @@ static void gsm0_receive(struct gsm_mux
 		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) {
 			gsm->state = GSM_FCS;
+		}
 		break;
 	case GSM_FCS:		/* FCS follows the packet */
 		gsm->received_fcs = c;
@@ -2055,7 +2059,7 @@ static void gsm1_receive(struct gsm_mux
 		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






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