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[esp32] Validate eFuse MAC reads and reject garbage MACs

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On some ESP32 boards (especially cheap clones), the eFuse custom MAC
area contains random garbage that passes the existing all-zeros/all-ones
validation. Additionally, esp_efuse_mac_get_default() can fail with CRC
errors, but the return value was being ignored, causing garbage MAC
addresses to be advertised via mDNS.

This caused Home Assistant to report false "MAC address changed" device
conflicts on every boot.

Two fixes:
- Check return values from eFuse MAC read functions and add a fallback
  chain: custom MAC -> default MAC -> raw eFuse bytes -> zeroed MAC.
- Reject multicast MACs (bit 0 of first byte set) in mac_address_is_valid()
  since device MACs must always be unicast.

Closes https://github.com/esphome/esphome/issues/14501
This commit is contained in:
J. Nick Koston
2026-03-20 21:55:06 -10:00
parent d203a46ef8
commit 935ab42b1e
2 changed files with 46 additions and 11 deletions
Download Patch File Download Diff File Expand all files Collapse all files

46
esphome/components/esp32/helpers.cpp
View File

@@ -9,11 +9,14 @@
#include <freertos/FreeRTOS.h>
#include <freertos/portmacro.h>
#include "esphome/core/log.h"
#include "esp_random.h"
#include "esp_system.h"
namespace esphome {
static const char *const TAG = "esp32";
bool random_bytes(uint8_t *data, size_t len) {
esp_fill_random(data, len);
return true;
@@ -63,22 +66,43 @@ LwIPLock::~LwIPLock() {
#endif
}
/// Read MAC and validate both the return code and content.
static bool read_valid_mac(uint8_t *mac, esp_err_t err) { return err == ESP_OK && mac_address_is_valid(mac); }
static constexpr size_t MAC_ADDRESS_SIZE_BITS = MAC_ADDRESS_SIZE * 8; // 48 bits
void get_mac_address_raw(uint8_t *mac) { // NOLINT(readability-non-const-parameter)
#if defined(CONFIG_SOC_IEEE802154_SUPPORTED)
// When CONFIG_SOC_IEEE802154_SUPPORTED is defined, esp_efuse_mac_get_default
// returns the 802.15.4 EUI-64 address, so we read directly from eFuse instead.
if (has_custom_mac_address()) {
esp_efuse_read_field_blob(ESP_EFUSE_MAC_CUSTOM, mac, 48);
} else {
esp_efuse_read_field_blob(ESP_EFUSE_MAC_FACTORY, mac, 48);
// Both paths already read raw eFuse bytes, so there is no CRC-bypass fallback
// (unlike the non-IEEE802154 path where esp_efuse_mac_get_default does CRC checks).
if (has_custom_mac_address() &&
read_valid_mac(mac, esp_efuse_read_field_blob(ESP_EFUSE_MAC_CUSTOM, mac, MAC_ADDRESS_SIZE_BITS))) {
return;
}
if (read_valid_mac(mac, esp_efuse_read_field_blob(ESP_EFUSE_MAC_FACTORY, mac, MAC_ADDRESS_SIZE_BITS))) {
return;
}
#else
if (has_custom_mac_address()) {
esp_efuse_mac_get_custom(mac);
} else {
esp_efuse_mac_get_default(mac);
if (has_custom_mac_address() && read_valid_mac(mac, esp_efuse_mac_get_custom(mac))) {
return;
}
if (read_valid_mac(mac, esp_efuse_mac_get_default(mac))) {
return;
}
// Default MAC read failed (e.g., eFuse CRC error) - try reading raw eFuse bytes
// directly, bypassing CRC validation. A MAC that passes mac_address_is_valid()
// (non-zero, non-broadcast, unicast) is almost certainly the real factory MAC
// with a corrupted CRC byte, which is far better than returning garbage or zeros.
if (read_valid_mac(mac, esp_efuse_read_field_blob(ESP_EFUSE_MAC_FACTORY, mac, MAC_ADDRESS_SIZE_BITS))) {
ESP_LOGW(TAG, "eFuse MAC CRC failed but raw bytes appear valid - using raw eFuse MAC");
return;
}
#endif
// All methods failed - zero the MAC rather than returning garbage
ESP_LOGE(TAG, "Failed to read a valid MAC address from eFuse");
memset(mac, 0, MAC_ADDRESS_SIZE);
}
void set_mac_address(uint8_t *mac) { esp_base_mac_addr_set(mac); }
@@ -88,9 +112,11 @@ bool has_custom_mac_address() {
uint8_t mac[6];
// do not use 'esp_efuse_mac_get_custom(mac)' because it drops an error in the logs whenever it fails
#ifndef USE_ESP32_VARIANT_ESP32
return (esp_efuse_read_field_blob(ESP_EFUSE_USER_DATA_MAC_CUSTOM, mac, 48) == ESP_OK) && mac_address_is_valid(mac);
return (esp_efuse_read_field_blob(ESP_EFUSE_USER_DATA_MAC_CUSTOM, mac, MAC_ADDRESS_SIZE_BITS) == ESP_OK) &&
mac_address_is_valid(mac);
#else
return (esp_efuse_read_field_blob(ESP_EFUSE_MAC_CUSTOM, mac, 48) == ESP_OK) && mac_address_is_valid(mac);
return (esp_efuse_read_field_blob(ESP_EFUSE_MAC_CUSTOM, mac, MAC_ADDRESS_SIZE_BITS) == ESP_OK) &&
mac_address_is_valid(mac);
#endif
#else
return false;

11
esphome/core/helpers.cpp
View File

@@ -863,7 +863,16 @@ bool mac_address_is_valid(const uint8_t *mac) {
is_all_ones = false;
}
}
return !(is_all_zeros || is_all_ones);
if (is_all_zeros || is_all_ones) {
return false;
}
// Reject multicast MACs (bit 0 of first byte set) - device MACs must be unicast.
// This catches garbage data from corrupted eFuse custom MAC areas, which often
// has random values that would otherwise pass the all-zeros/all-ones check.
if (mac[0] & 0x01) {
return false;
}
return true;
}
void IRAM_ATTR HOT delay_microseconds_safe(uint32_t us) {