#3716, #3707: Fix native-cipher segfault when using musl libc

native/compile-native.sh

@@ -2,8 +2,10 @@
 
 set -eu
 
+CWD=$(pwd)
+
 echo "Compiling mbedtls"
-(cd mbedtls && make no_test)
+(cd mbedtls && CFLAGS="-fPIC -I$CWD/src/main/c -DMBEDTLS_USER_CONFIG_FILE='<mbedtls_custom_config.h>'" make no_test)
 
 echo "Compiling zlib"
 (cd zlib && CFLAGS=-fPIC ./configure --static && make)

native/src/main/c/NativeCipherImpl.c

@@ -5,11 +5,15 @@
 #include "shared.h"
 #include "net_md_5_bungee_jni_cipher_NativeCipherImpl.h"
 
+// Hack to keep the compiler from optimizing the memset away
+static void *(*const volatile memset_func)(void *, int, size_t) = memset;
+
 typedef unsigned char byte;
 
 typedef struct crypto_context {
     int mode;
     mbedtls_aes_context cipher;
+    int keyLen;
     byte key[];
 } crypto_context;
 
@@ -22,6 +26,7 @@ jlong JNICALL Java_net_md_15_bungee_jni_cipher_NativeCipherImpl_init(JNIEnv* env
         return 0;
     }
 
+    crypto->keyLen = (int) keyLen;
     (*env)->GetByteArrayRegion(env, key, 0, keyLen, (jbyte*) &crypto->key);
 
     mbedtls_aes_init(&crypto->cipher);
@@ -36,6 +41,7 @@ void Java_net_md_15_bungee_jni_cipher_NativeCipherImpl_free(JNIEnv* env, jobject
     crypto_context *crypto = (crypto_context*) ctx;
 
     mbedtls_aes_free(&crypto->cipher);
+    memset_func(crypto->key, 0, (size_t) crypto->keyLen);
     free(crypto);
 }
 

native/src/main/c/mbedtls_custom_config.h

@@ -0,0 +1,31 @@
+
+// This is a hack to deal with a glitch that happens when mbedtls is compiled against glibc
+// but then run on a linux distro that uses musl libc. This implementation of the zeroize
+// is compatible with both glibc and musl without requiring the library to be recompiled.
+
+// I checked with a disassembler and for BungeeCord's usage of the library, implementing
+// this function as a static function only resulted in 2 different subroutines referencing
+// different versions of memset_func, so we might as well keep things simple and use a
+// static function here instead of requiring the mbedtls makefile to be modified to add
+// additional source files.
+
+#ifndef _INCLUDE_MBEDTLS_CUSTOM_CONFIG_H
+#define _INCLUDE_MBEDTLS_CUSTOM_CONFIG_H
+
+#include <string.h>
+
+#define MBEDTLS_PLATFORM_ZEROIZE_ALT
+
+#define mbedtls_platform_zeroize mbedtls_platform_zeroize_impl
+
+// hack to prevent compilers from optimizing the memset away
+static void *(*const volatile memset_func)(void *, int, size_t) = memset;
+
+static void mbedtls_platform_zeroize_impl(void *buf, size_t len) {
+    if (len > 0) {
+        memset_func(buf, 0, len);
+    }
+}
+
+#endif // _INCLUDE_MBEDTLS_CUSTOM_CONFIG_H
+