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can: gw: fix OOB heap access in cgw_csum_crc8_rel()

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cgw_csum_crc8_rel() correctly computes bounds-safe indices via calc_idx():

    int from = calc_idx(crc8->from_idx, cf->len);
    int to   = calc_idx(crc8->to_idx,   cf->len);
    int res  = calc_idx(crc8->result_idx, cf->len);

    if (from < 0 || to < 0 || res < 0)
        return;

However, the loop and the result write then use the raw s8 fields directly
instead of the computed variables:

    for (i = crc8->from_idx; ...)        /* BUG: raw negative index */
    cf->data[crc8->result_idx] = ...;    /* BUG: raw negative index */

With from_idx = to_idx = result_idx = -64 on a 64-byte CAN FD frame,
calc_idx(-64, 64) = 0 so the guard passes, but the loop iterates with
i = -64, reading cf->data[-64], and the write goes to cf->data[-64].
This write might end up to 56 (7.0-rc) or 40 (<= 6.19) bytes before the
start of the canfd_frame on the heap.

The companion function cgw_csum_xor_rel() uses `from`/`to`/`res`
correctly throughout; fix cgw_csum_crc8_rel() to match.

Confirmed with KASAN on linux-7.0-rc2:
  BUG: KASAN: slab-out-of-bounds in cgw_csum_crc8_rel+0x515/0x5b0
  Read of size 1 at addr ffff8880076619c8 by task poc_cgw_oob/62

To configure the can-gw crc8 checksums CAP_NET_ADMIN is needed.

Fixes: 456a8a646b ("can: gw: add support for CAN FD frames")
Cc: stable@vger.kernel.org
Reported-by: Ali Norouzi <ali.norouzi@keysight.com>
Reviewed-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Ali Norouzi <ali.norouzi@keysight.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20260319-fix-can-gw-and-can-isotp-v2-1-c45d52c6d2d8@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
This commit is contained in:
Ali Norouzi
2026-03-19 16:47:44 +01:00
committed by Marc Kleine-Budde
parent 46eee1661a
commit b9c310d727
1 changed files with 3 additions and 3 deletions
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6
net/can/gw.c
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@@ -375,10 +375,10 @@ static void cgw_csum_crc8_rel(struct canfd_frame *cf,
return;
if (from <= to) {
for (i = crc8->from_idx; i <= crc8->to_idx; i++)
for (i = from; i <= to; i++)
crc = crc8->crctab[crc ^ cf->data[i]];
} else {
for (i = crc8->from_idx; i >= crc8->to_idx; i--)
for (i = from; i >= to; i--)
crc = crc8->crctab[crc ^ cf->data[i]];
}
@@ -397,7 +397,7 @@ static void cgw_csum_crc8_rel(struct canfd_frame *cf,
break;
}
cf->data[crc8->result_idx] = crc ^ crc8->final_xor_val;
cf->data[res] = crc ^ crc8->final_xor_val;
}
static void cgw_csum_crc8_pos(struct canfd_frame *cf,