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

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-9/-/merge_requests/4368

JIRA: https://issues.redhat.com/browse/RHEL-39354  
CVE: CVE-2024-36016  
  
Build Info: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=61572658  
Tested: Did sanity boot testing Intel (intel-arrowlake-s-02) system.  
  
commit 47388e8  
Author: Daniel Starke <daniel.starke@siemens.com>  
Date:   Wed Apr 24 07:48:41 2024 +0200  
  
    tty: n_gsm: fix possible out-of-bounds in gsm0_receive()  
  
    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>  
  
Signed-off-by: Steve Best <sbest@redhat.com>

Approved-by: John W. Linville <linville@redhat.com>
Approved-by: David Arcari <darcari@redhat.com>
Approved-by: Tony Camuso <tcamuso@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: Lucas Zampieri <lzampier@redhat.com>

Author: zampierilucas

drivers/tty/n_gsm.c

@@ -2913,7 +2913,10 @@ 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,
@@ -3026,7 +3029,7 @@ 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