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

[Jeremy Kerr]

Hi Doruk,

Thanks for the report. The analysis looks solid, but I do have a
recommendation for a different fix. One comment inline too.

> 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.
> 
> Reject a zero-length frame in the header parser, matching the existing
> upper-bound rejection.
> 
> KASAN, on a frame of 0x7e 0x01 0x00 followed by data bytes:
> 
>   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
> 
> Found by 0sec (https://0sec.ai).
> Fixes: a0c2ccd9b5ad ("mctp: Add MCTP-over-serial transport binding")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: 0sec

I assume this needs to be in the Assisted-by format.

> Signed-off-by: Doruk Tan Ozturk <doruk@xxxxxxx>
> ---
>  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..1e3d285c0500 100644
> --- a/drivers/net/mctp/mctp-serial.c
> +++ b/drivers/net/mctp/mctp-serial.c
> @@ -313,7 +313,7 @@ static void mctp_serial_push_header(struct mctp_serial *dev, u8 c)
>  		}
>  		break;
>  	case 2:
> -		if (c > MCTP_SERIAL_FRAME_MTU) {
> +		if (c == 0 || c > MCTP_SERIAL_FRAME_MTU) {
>  			dev->rxstate = STATE_ERR;
>  		} else {
>  			dev->rxlen = c;

We probably want to advance directly to STATE_TRAILER instead, in order
to consume the trailer and framing bytes for cases when it's a
legitimate (although somewhat useless) zero-length frame.

Perhaps:

--- 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;

We'll still land in STATE_ERROR on incorrect framing, but just during
trailer parse instead.

We could then add an early error path rather than creating a zero-length
skb (which then gets rejected by the MCTP core), but that would be best
done separately.

Cheers,


Jeremy