SSL validation using root cert crashes official `HTTPSRequestCACertAxTLS` example

[nagimov]

Basic Infos

• This issue complies with the issue POLICY doc.
• I have read the documentation at readthedocs and the issue is not addressed there.
• I have tested that the issue is present in current master branch (aka latest git).
• I have searched the issue tracker for a similar issue.
• If there is a stack dump, I have decoded it.
• I have filled out all fields below.

Platform

• Hardware: ESP8266EX
• Core Version: 2.5.2
• Development Env: Arduino IDE
• Operating System: Ubuntu

Settings in IDE

• Module: Nodemcu
• Flash Mode: DOUT
• Flash Size: 4MB
• lwip Variant: v2 Lower Memory
• Reset Method: ck
• Flash Frequency: 40Mhz
• CPU Frequency: 80Mhz
• Upload Using: SERIAL (USB CH340 chip)
• Upload Speed: 115200

Problem Description

Official example sketch HTTPSRequestCACertAxTLS.ino causes ESP to crash with exception on ssl_verify_cert().

Steps reproducing the issue:

• git clone https://github.com/esp8266/Arduino
• cd Arduino/libraries/ESP8266WiFi/examples/HTTPSRequestCACertAxTLS/
• sed -i "s/your-ssid/my-ssid/" HTTPSRequestCACertAxTLS.ino
• sed -i "s/your-password/my-password/" HTTPSRequestCACertAxTLS.ino
• arduino HTTPSRequestCACertAxTLS.ino
• upload sketch

Crashes with MWE as well (root certificate must be defined in CACert.ino):

MCVE Sketch

#define USING_AXTLS
#include <time.h>
#include <ESP8266WiFi.h>

#include <WiFiClientSecureAxTLS.h>
using namespace axTLS;

#ifndef STASSID
#define STASSID "my-ssid"
#define STAPSK  "my-password"
#endif

const char* ssid = STASSID;
const char* password = STAPSK;

const char* host = "api.github.com";
const int httpsPort = 443;

// Root certificate used by api.github.com.
// Defined in "CACert" tab.
extern const unsigned char caCert[] PROGMEM;
extern const unsigned int caCertLen;

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored  "-Wdeprecated-declarations"
WiFiClientSecure client;
#pragma GCC diagnostic pop

void setup() {
  Serial.begin(115200);
  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
  }

  // Synchronize time useing SNTP
  configTime(8 * 3600, 0, "pool.ntp.org", "time.nist.gov");
  time_t now = time(nullptr);
  while (now < 8 * 3600 * 2) {
    delay(500);
    now = time(nullptr);
  }
  struct tm timeinfo;
  gmtime_r(&now, &timeinfo);

  // Load root certificate in DER format into WiFiClientSecure object
  bool res = client.setCACert_P(caCert, caCertLen);
  if (!res) {
    Serial.println("Failed to load root CA certificate!");
    while (true) {
      yield();
    }
  }
}

void loop() {
  if (!client.connect(host, httpsPort)) {
    Serial.println("connection failed");
    return;
  }
  Serial.println("connected");
  // Verify validity of server's certificate  <<<<==== crashes here
  if (client.verifyCertChain(host)) {
    Serial.println("Server certificate verified");
  } else {
    Serial.println("ERROR: certificate verification failed!");
    return;
  }
  Serial.println("verified");
}

Debug Messages

Stack trace from MWE:

connected

Exception (3):
epc1=0x4021b7ba epc2=0x00000000 epc3=0x00000000 excvaddr=0x4024bcee depc=0x00000000

>>>stack>>>

ctx: cont
sp: 3ffffd60 end: 3fffffc0 offset: 01a0
3fffff00:  5d1fa3e7 000c2c7f 3fffff20 3ffef564  
3fffff10:  3fff31ec 3fff0a14 3fff33c4 3fff03cc  
3fffff20:  3fffff60 3ffef734 3fff0a9c 40204aae  
3fffff30:  3ffe8616 00000000 3ffe8615 3ffee964  
3fffff40:  3ffee898 00000009 3ffee8fc 3ffee964  
3fffff50:  3ffee898 3fff0ed4 3ffef6c4 402196af  
3fffff60:  00000000 3ffee898 3ffe865b 402033bd  
3fffff70:  3ffe860d 3ffee898 3ffef6c4 4020270c  
3fffff80:  3ffe865b 0572528c 3ffee8fc 40203414  
3fffff90:  3ffee898 3ffe851c 3ffee8fc 4020114e  
3fffffa0:  3fffdad0 00000000 3ffee934 40203bf0  
3fffffb0:  feefeffe feefeffe 3ffe8554 40100459  
<<<stack<<<

 ets Jan  8 2013,rst cause:2, boot mode:(3,6)

load 0x4010f000, len 1384, room 16 
tail 8
chksum 0x2d
csum 0x2d
v8b899c12
~ld

Decoded:

PC: 0x4021b7ba: x509_verify at ssl/x509.c line 437
EXCVADDR: 0x4024bcee

Decoding stack results
0x40204aae: uart_write(uart_t*, char const*, size_t) at /home/nagimov/.arduino15/packages/esp8266/hardware/esp8266/2.5.2/cores/esp8266/uart.cpp line 498
0x402196af: ssl_verify_cert at ssl/tls1.c line 2101
0x402033bd: Print::write(char const*) at /home/nagimov/.arduino15/packages/esp8266/hardware/esp8266/2.5.2/cores/esp8266/Print.h line 60
0x4020270c: axTLS::WiFiClientSecure::verifyCertChain(char const*) at /home/nagimov/.arduino15/packages/esp8266/hardware/esp8266/2.5.2/libraries/ESP8266WiFi/src/include/SSLContext.h line 323
0x40203414: Print::println(char const*) at /home/nagimov/.arduino15/packages/esp8266/hardware/esp8266/2.5.2/cores/esp8266/Print.cpp line 190
0x4020114e: loop() at /home/nagimov/esp/Arduino/libraries/ESP8266WiFi/examples/HTTPSRequestCACertAxTLS/HTTPSRequestCACertAxTLS.ino line 64
0x40203bf0: loop_wrapper() at /home/nagimov/.arduino15/packages/esp8266/hardware/esp8266/2.5.2/cores/esp8266/core_esp8266_main.cpp line 125

[earlephilhower]

Axtls is relying on the non-32b aligned read exception handler in SDK3.0. But we had to move back to 2.2.x since pre3.0 :

#ifdef CONFIG_SSL_CERT_VERIFICATION
static const uint8_t sig_prefix_md5[] PROGMEM = {0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00, 0x04, 0x10};
static const uint8_t sig_prefix_sha1[] PROGMEM = {0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E, 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14};
static const uint8_t sig_prefix_sha256[] PROGMEM = {0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20};
static const uint8_t sig_prefix_sha384[] PROGMEM = {0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05, 0x00, 0x04, 0x30};
static const uint8_t sig_prefix_sha512[] PROGMEM = {0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05, 0x00, 0x04, 0x40};

/**
 * Take a signature and decrypt it.
 */
static bigint *sig_verify(BI_CTX *ctx, const uint8_t *sig, int sig_len, uint8_t sig_type,
        bigint *modulus, bigint *pub_exp)
{
    int i;
    bigint *decrypted_bi, *dat_bi;
    bigint *bir = NULL;
    uint8_t *block = (uint8_t *)malloc(sig_len);

    const uint8_t *sig_prefix = NULL;
    uint8_t sig_prefix_size = 0, hash_len = 0;
    /* adjust our expections */
    switch (sig_type)
    {
        case SIG_TYPE_MD5:
            sig_prefix = sig_prefix_md5;
            sig_prefix_size = sizeof(sig_prefix_md5);
        break;
        case SIG_TYPE_SHA1:
....
        break;
    }
    if (sig_prefix)
        hash_len = sig_prefix[sig_prefix_size - 1];

So reading the last byte (to get the len) and set hash_len causes an exception.

[nagimov]

Confirming working ok with current master branch. Awesome quick turnaround, thanks!