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

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@gmail.com
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218708
Tested-by: j51569436@gmail.com
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/20240424054842.7741-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index c20f69a4c5e9e79db326eb0d39d1d8bed732eab4..0a367fa23c277749efe3dba9e665ba91497cc068 100644 (file)
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2082,8 +2082,12 @@ 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) {
                        gsm->state = GSM_FCS;
+               }
                break;
        case GSM_FCS:           /* FCS follows the packet */
                gsm->received_fcs = c;
@@ -2176,7 +2180,7 @@ 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