Fix issue with MemSan and entropy

Due to the recent change about entropy sources strength, it is no longer
acceptable to just disable the platform source. So, instead "fix" it so that
it is clear to MemSan that memory is initialized.

I tried __attribute__((no_sanitize_memory)) and MemSan's blacklist file, but
couldn't seem to get them to work.

library/entropy_poll.c

@@ -81,8 +81,16 @@ int mbedtls_platform_entropy_poll( void *data, unsigned char *output, size_t len
 #include <sys/syscall.h>
 #if defined(SYS_getrandom)
 #define HAVE_GETRANDOM
+
 static int getrandom_wrapper( void *buf, size_t buflen, unsigned int flags )
 {
+    /* MemSan cannot understand that the syscall writes to the buffer */
+#if defined(__has_feature)
+#if __has_feature(memory_sanitizer)
+    memset( buf, 0, buflen );
+#endif
+#endif
+
     return( syscall( SYS_getrandom, buf, buflen, flags ) );
 }
 

tests/scripts/all.sh

@@ -247,7 +247,6 @@ msg "build: MSan (clang)" # ~ 1 min 20s
 cleanup
 cp "$CONFIG_H" "$CONFIG_BAK"
 scripts/config.pl unset MBEDTLS_AESNI_C # memsan doesn't grok asm
-scripts/config.pl set MBEDTLS_NO_PLATFORM_ENTROPY # memsan vs getrandom()
 CC=clang cmake -D CMAKE_BUILD_TYPE:String=MemSan .
 make