Re: [PATCH net v2] can: can327: Fix out-of-bounds write in can327_parse_frame()

[Marc Kleine-Budde]

On 19.08.2026 07:18:18, Baul Lee wrote:
> can327_parse_frame() assigns the CAN payload length from the DLC nibble
> of the adapter's ASCII frame line, hex_to_bin(elm->rxbuf[datastart - 2]),
> without validating it. A standard-format line only has to satisfy
> rxbuf[3] == ' ' and rxbuf[5] == ' ', so the DLC nibble rxbuf[4] can be a
> space, for which hex_to_bin() returns -1, and that becomes 255 in the u8
> frame->len. A hex nibble of 9 to f is not rejected either, while
> CAN_MAX_DLEN is 8.
>
> frame->data[] is the 8-byte payload of the 16-byte struct can_frame
> returned by alloc_can_skb(), so the data-nibble loop writes up to 255
> device-controlled bytes, 247 of them past the frame and over the
> trailing skb_shared_info. The length check before the loop only requires
> the line to be frame->len * 3 + datastart bytes, which a long enough
> line of hex and spaces satisfies. Freeing the corrupted skb then faults:
>
>   pc : skb_release_data+0xf4/0x200
>   Call trace:
>    skb_release_data+0xf4/0x200 (P)
>    sk_skb_reason_drop+0x40/0xa4
>    can_rcv+0x6c/0xbc
>    __netif_receive_skb_one_core+0x40/0x4c
>    can327_ldisc_rx+0xc8/0x140
>    tty_ldisc_receive_buf+0x48/0x60
>    flush_to_ldisc+0xdc/0x1b0
>   Kernel panic - not syncing: Oops: Fatal exception in interrupt
>
> Reject the line when the nibble is not a hex digit or exceeds
> CAN_MAX_DLEN, as the parser already does for other malformed lines.
>
> Attaching the N_CAN327 line discipline requires CAP_NET_ADMIN, but the
> frame lines then come from the ELM327 device, so a malicious adapter
> reaches this path with device-controlled data.
>
> Discovered by XBOW, triaged by Baul Lee <baul.lee@xxxxxxxx>
>
> Fixes: 43da2f07622f ("can: can327: CAN/ldisc driver for ELM327 based OBD-II adapters")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Baul Lee <baul.lee@xxxxxxxx>
> Reviewed-by: Max Staudt <max@xxxxxxxxx>

Nitpick, your S-o-b must come last.

regards,
Marc

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