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gcoap: fix underflow when correcting ETag from cache [backport 2023.10] #19987

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Oct 19, 2023
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gcoap: fix underflow when correcting ETag from cache [backport 2023.10] #19987

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16 changes: 14 additions & 2 deletions sys/net/application_layer/gcoap/gcoap.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1331,8 +1331,21 @@ static ssize_t _cache_check(const uint8_t *buf, size_t len,
if ((resp_etag_len > 0) && ((size_t)resp_etag_len <= COAP_ETAG_LENGTH_MAX)) {
uint8_t *tmp_etag;
ssize_t tmp_etag_len = coap_opt_get_opaque(&req, COAP_OPT_ETAG, &tmp_etag);

if (tmp_etag_len >= resp_etag_len) {
/* peak length without padding */
size_t rem_len = (len - (tmp_etag + tmp_etag_len - buf));

if ((tmp_etag < buf) || (tmp_etag > (buf + len)) ||
(rem_len > (len - ((tmp_etag + COAP_ETAG_LENGTH_MAX) - buf)))) {
DEBUG("gcoap: invalid calculated padding length (%lu) for ETag injection "
"during cache lookup.\n", (long unsigned)rem_len);
/* something fishy happened in the request. Better don't return cache entry */
*cache_hit = false;
#if IS_USED(MODULE_NANOCOAP_CACHE)
memset(memo->cache_key, 0, sizeof(memo->cache_key));
#endif
return -EINVAL;
}
memcpy(tmp_etag, resp_etag, resp_etag_len);
/* shorten ETag option if necessary */
if ((size_t)resp_etag_len < COAP_ETAG_LENGTH_MAX) {
Expand All @@ -1345,7 +1358,6 @@ static ssize_t _cache_check(const uint8_t *buf, size_t len,
* bitmask resp_etag_len */
*start |= (uint8_t)resp_etag_len;
/* remove padding */
size_t rem_len = (len - (tmp_etag + COAP_ETAG_LENGTH_MAX - buf));
memmove(tmp_etag + resp_etag_len, tmp_etag + COAP_ETAG_LENGTH_MAX, rem_len);
len -= (COAP_ETAG_LENGTH_MAX - resp_etag_len);
}
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