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

Overview

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

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

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.

Affected

• Linux >= 456a8a646b2563438c16a9b27decf9aa717f1ebb < e7c99348b0612b2bc02d5ce6ff9873261cc7605f
• Linux >= 456a8a646b2563438c16a9b27decf9aa717f1ebb < 999ca48d55a8a46da21519db7e834e5867200379
• Linux >= 456a8a646b2563438c16a9b27decf9aa717f1ebb < a025283d7f7404c739225e457fb99db2368bb544
• Linux >= 456a8a646b2563438c16a9b27decf9aa717f1ebb < 54ecdf76a55e75c1f5085e440f8ab671a3283ef5
• Linux >= 456a8a646b2563438c16a9b27decf9aa717f1ebb < c4e8eaa75fa0b6bcbfa5356d6195c4ad0e05e57a
• Linux >= 456a8a646b2563438c16a9b27decf9aa717f1ebb < 84f8b76d24273175a22713e83e90874e1880d801
• Linux >= 456a8a646b2563438c16a9b27decf9aa717f1ebb < 66b689efd08227da2c5ca49b58b30a95d23c695a
• Linux >= 456a8a646b2563438c16a9b27decf9aa717f1ebb < b9c310d72783cc2f30d103eed83920a5a29c671a
• Linux 5.4

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.