CVE-2026-68124: mctp: serial: handle zero-length frames to prevent rx buffer overflow

In the Linux kernel, the following vulnerability has been resolved:

mctp: serial: handle zero-length frames to prevent rx buffer overflow

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

if (c > MCTPSERIALFRAMEMTU) { dev->rxstate = STATEERR; } else { dev->rxlen = c; dev->rxpos = 0; dev->rxstate = STATEDATA; ... }

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

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

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 CAPNETADMIN to attach the NMCTP 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 STATETRAILER instead of STATEDATA. 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 STATEDATA 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 mctpserialttyreceivebuf Write of size 1 at addr ... by task kworker/u16:0 mctpserialttyreceivebuf ttyldiscreceivebuf flushtoldisc Allocated by task 152: allocnetdevmqs mctpserialopen

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

Found by 0sec automated security-research tooling (https://0sec.ai).