Re: [PATCH net v2] mctp: serial: handle zero-length frames to prevent rx buffer overflow

[Simon Horman]

On Wed, Jul 15, 2026 at 10:20:21AM +0200, Doruk Tan Ozturk wrote:
> 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@xxxxxxxxxxxxxxx
> Suggested-by: Jeremy Kerr <jk@xxxxxxxxxxxxxxxxxxxx>
> Assisted-by: 0sec:multi-model
> Signed-off-by: Doruk Tan Ozturk <doruk@xxxxxxx>

Reviewed-by: Simon Horman <horms@xxxxxxxxxx>