The MCTP serial receive state machine reads a frame length byte in
mctp_serial_push_header() case 2 and validates it upper-bound-only:

	if (c > MCTP_SERIAL_FRAME_MTU) {
		dev->rxstate = STATE_ERR;
	} else {
		dev->rxlen = c;
		dev->rxpos = 0;
		dev->rxstate = STATE_DATA;
		...
	}

A length of zero passes this check, so rxlen is set to 0 and the state
machine advances to STATE_DATA. In mctp_serial_push() STATE_DATA, the
incoming byte is stored and rxpos incremented before the terminator is
tested:

	dev->rxbuf[dev->rxpos] = c;
	dev->rxpos++;
	dev->rxstate = STATE_DATA;
	if (dev->rxpos == dev->rxlen) {
		dev->rxpos = 0;
		dev->rxstate = STATE_TRAILER;
	}

With rxlen == 0 the "rxpos == rxlen" terminator can never fire (rxpos is
already 1 on the first data byte), so subsequent bytes are written past
the end of the fixed 74-byte rxbuf, which is the last member of the
netdev private area. Every following data byte is an attacker-controlled
1-byte out-of-bounds heap write, and the overflow continues until a
frame (0x7e) or escape byte resets the parser -- effectively unbounded.

Reaching this requires CAP_NET_ADMIN to attach the N_MCTP line
discipline and bring the resulting mctpserialN netdev up, after which
the bytes arrive via the tty receive path.

Route a zero-length frame straight to STATE_TRAILER instead of
STATE_DATA. The trailer/framing bytes are still consumed, and the frame
resolves to a zero-length skb that the MCTP core rejects; the parser
never enters STATE_DATA with rxlen == 0, so the out-of-bounds write can
no longer occur.

KASAN, on a frame of 0x7e 0x01 0x00 followed by data bytes (before this
change):

  UBSAN: array-index-out-of-bounds in drivers/net/mctp/mctp-serial.c:370
  index 74 is out of range for type 'u8 [74]'
  BUG: KASAN: slab-out-of-bounds in mctp_serial_tty_receive_buf
  Write of size 1 at addr ... by task kworker/u16:0
   mctp_serial_tty_receive_buf
   tty_ldisc_receive_buf
   flush_to_ldisc
  Allocated by task 152:
   alloc_netdev_mqs
   mctp_serial_open

v2: route zero-length frames to STATE_TRAILER instead of STATE_ERR so
    the trailer/framing bytes are still consumed (Jeremy Kerr).

Found by 0sec automated security-research tooling (https://0sec.ai).
Fixes: a0c2ccd9b5ad ("mctp: Add MCTP-over-serial transport binding")
Cc: stable@vger.kernel.org
Suggested-by: Jeremy Kerr <jk@codeconstruct.com.au>
Assisted-by: 0sec:multi-model
Signed-off-by: Doruk Tan Ozturk <doruk@0sec.ai>
---
 drivers/net/mctp/mctp-serial.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/mctp/mctp-serial.c b/drivers/net/mctp/mctp-serial.c
index 26c9a33fd636..a5070ffa9a95 100644
--- a/drivers/net/mctp/mctp-serial.c
+++ b/drivers/net/mctp/mctp-serial.c
@@ -318,7 +318,7 @@ static void mctp_serial_push_header(struct mctp_serial *dev, u8 c)
 		} else {
 			dev->rxlen = c;
 			dev->rxpos = 0;
-			dev->rxstate = STATE_DATA;
+			dev->rxstate = c > 0 ? STATE_DATA : STATE_TRAILER;
 			dev->rxfcs = crc_ccitt_byte(dev->rxfcs, c);
 		}
 		break;
-- 
2.43.0