OpenSSL 3.5.4: Data Race In Ossl_default_provider_init

by SLV Team 55 views

Hey everyone,

I wanted to bring up a potential issue I've encountered while working with OpenSSL 3.5.4, specifically when it's built with -fsanitize=thread. It seems there's a data race condition lurking within the ossl_default_provider_init function. Let's dive into the details and see if we can figure out a solid solution.

The Problem: Concurrent Access to Global Variables

The core of the issue lies in how ossl_default_provider_init handles the global variables c_get_params and c_gettable_params. The function appears to unconditionally overwrite these globals, which becomes problematic when OSSL_PROVIDER_load is invoked simultaneously by multiple threads. This is a classic recipe for a data race, where multiple threads try to modify the same memory location without proper synchronization.

Looking at the source code (https://github.com/openssl/openssl/blob/5aaf9746f649d15b3ad006acdc6a958819acc536/providers/defltprov.c#L765-L768), it's clear that these global variables are indeed being modified. The question then becomes: how do we prevent these concurrent modifications from causing problems?

Why This Happens: Multi-Threaded OSSL_PROVIDER_load

In my particular application, I'm using a separate OSSL_LIB_CTX for each thread. This is a deliberate design choice to minimize contention and improve overall performance. However, this approach leads to each thread calling OSSL_PROVIDER_load to load the "default" provider into its respective context. Because each thread initializes the default provider and thus calls the ossl_default_provider_init function, this triggers the data race.

The Evidence: ThreadSanitizer Report

Here's a snippet from the ThreadSanitizer report that clearly demonstrates the data race:

WARNING: ThreadSanitizer: data race (pid=1810)
 Write of size 8 at 0x7ff0d5f0a540 by thread T3:
 #0 ossl_default_provider_init openssl-3.5.4/providers/defltprov.c:737:31 (libcrypto.so.3+0x5b7450)
 #1 provider_init openssl-3.5.4/crypto/provider_core.c:1045:10 (libcrypto.so.3+0x3ea936)
 #2 provider_activate openssl-3.5.4/crypto/provider_core.c:1271:14 (libcrypto.so.3+0x3ea936)
 #3 ossl_provider_activate openssl-3.5.4/crypto/provider_core.c:1412:18 (libcrypto.so.3+0x3ea552)
 #4 OSSL_PROVIDER_try_load_ex openssl-3.5.4/crypto/provider.c:31:10 (libcrypto.so.3+0x3e6c4e)
 #5 OSSL_PROVIDER_load_ex openssl-3.5.4/crypto/provider.c:62:16 (libcrypto.so.3+0x3e6dc0)
 #6 OSSL_PROVIDER_load openssl-3.5.4/crypto/provider.c:68:12 (libcrypto.so.3+0x3e6dc0)
 ...

 Previous write of size 8 at 0x7ff0d5f0a540 by thread T2:
 #0 ossl_default_provider_init openssl-3.5.4/providers/defltprov.c:737:31 (libcrypto.so.3+0x5b7450)
 #1 provider_init openssl-3.5.4/crypto/provider_core.c:1045:10 (libcrypto.so.3+0x3ea936)
 #2 provider_activate openssl-3.5.4/crypto/provider_core.c:1271:14 (libcrypto.so.3+0x3ea936)
 #3 ossl_provider_activate openssl-3.5.4/crypto/provider_core.c:1412:18 (libcrypto.so.3+0x3ea552)
 #4 OSSL_PROVIDER_try_load_ex openssl-3.5.4/crypto/provider.c:31:10 (libcrypto.so.3+0x3e6c4e)
 #5 OSSL_PROVIDER_load_ex openssl-3.5.4/crypto/provider.c:62:16 (libcrypto.so.3+0x3e6dc0)
 #6 OSSL_PROVIDER_load openssl-3.5.4/crypto/provider.c:68:12 (libcrypto.so.3+0x3e6dc0)
 ...

The report clearly indicates that threads T2 and T3 are both attempting to write to the same memory location (0x7ff0d5f0a540) within ossl_default_provider_init, leading to the data race.

Potential Solution: Local Variables?

One potential solution that comes to mind is to change c_get_params and c_gettable_params to be local variables within ossl_default_provider_init. This would eliminate the shared state and prevent the data race. However, I'm not entirely sure if this change would have any unintended consequences elsewhere in the code. It's crucial to understand the scope and usage of these variables before making such a modification.

Other Possible Solutions:

  1. Mutex: Implement a mutex lock around the critical section in ossl_default_provider_init where c_get_params and c_gettable_params are being written to. This would serialize access to these variables, preventing concurrent writes.
  2. Double-Checked Locking: Employ a double-checked locking pattern to ensure that the initialization of c_get_params and c_gettable_params happens only once. This can improve performance compared to always acquiring a mutex.
  3. Thread-Local Storage: Use thread-local storage for c_get_params and c_gettable_params. This would give each thread its own private copy of these variables, eliminating the shared state and the data race.

Request for Feedback

I'd love to hear your thoughts on this issue. Has anyone else encountered this data race? Do you have any insights into the potential consequences of making c_get_params and c_gettable_params local variables? Are there any other potential solutions that I haven't considered? Let's discuss!

More Detailed Explanation and Context

To provide a more comprehensive understanding of the issue, let's delve deeper into the context and implications of this data race. The ossl_default_provider_init function is responsible for initializing the default provider in OpenSSL. This provider offers a wide range of cryptographic algorithms and functionalities that are essential for many applications.

When OSSL_PROVIDER_load is called, it triggers the initialization of the specified provider. In a multi-threaded environment, multiple threads might attempt to load the default provider concurrently. This leads to multiple calls to ossl_default_provider_init, which in turn causes the data race on the global variables c_get_params and c_gettable_params.

The consequences of this data race can be unpredictable and potentially severe. It could lead to memory corruption, incorrect cryptographic operations, or even application crashes. Therefore, it's crucial to address this issue promptly and effectively.

Analyzing the Code

Let's take a closer look at the relevant code snippet from providers/defltprov.c:

static OSSL_PARAM deflt_gettable_params[] = {
    OSSL_PARAM_DEFN(OSSL_PROV_PARAM_NAME, OSSL_PARAM_UTF8_STRING, NULL, 0),
    OSSL_PARAM_DEFN(OSSL_PROV_PARAM_VERSION, OSSL_PARAM_UTF8_STRING, NULL, 0),
    OSSL_PARAM_DEFN(OSSL_PROV_PARAM_BUILDINFO, OSSL_PARAM_UTF8_STRING, NULL, 0),
    OSSL_PARAM_DEFN(OSSL_PROV_PARAM_STATUS, OSSL_PARAM_INTEGER, NULL, 0),
    OSSL_PARAM_END
};

static const OSSL_PARAM *ossl_default_gettable_params(void *provctx)
{
    return deflt_gettable_params;
}

static OSSL_PARAM deflt_get_params[] = {
    OSSL_PARAM_DEFN(OSSL_PROV_PARAM_NAME, OSSL_PARAM_UTF8_STRING, NULL, 0),
    OSSL_PARAM_DEFN(OSSL_PROV_PARAM_VERSION, OSSL_PARAM_UTF8_STRING, NULL, 0),
    OSSL_PARAM_DEFN(OSSL_PROV_PARAM_BUILDINFO, OSSL_PARAM_UTF8_STRING, NULL, 0),
    OSSL_PARAM_DEFN(OSSL_PROV_PARAM_STATUS, OSSL_PARAM_INTEGER, NULL, 0),
    OSSL_PARAM_END
};

static const OSSL_PARAM *ossl_default_get_params(void *provctx)
{
    return deflt_get_params;
}

static OSSL_PROVIDER_INIT_fn ossl_default_provider_init;

static const OSSL_DISPATCH deflt_dispatch[] = {
    { OSSL_FUNC_PROVIDER_INIT, (void (*)(void))ossl_default_provider_init },
    { 0, NULL }
};

static int ossl_default_provider_init(OSSL_PROVIDER *prov, const OSSL_PARAM params[])
{
    OSSL_LIB_CTX *libctx = OSSL_PROVIDER_get_libctx(prov);

    if (libctx == NULL)
    {
        printf("libctx == NULL\n");
        return 0;
    }

    OSSL_PROVIDER_set_name(prov, "OpenSSL Default Provider");

    /*
     * The OpenSSL 3.0 code initializes these in the OPENSSL_init_crypto() function.
     * That function is called before any threads are created. So it is thread safe.
     */
    c_get_params = ossl_default_get_params;
    c_gettable_params = ossl_default_gettable_params;

    if (!OSSL_PROVIDER_add_builtin(prov, libctx, "algprops", ossl_algorithm_properties, params))
    {
        printf("Failed to add algprops\n");
        return 0;
    }

    if (!OSSL_PROVIDER_add_builtin(prov, libctx, "fetchprops", ossl_fetchable_properties, params))
    {
        printf("Failed to add fetchprops\n");
        return 0;
    }

    return 1;
}

As you can see, the ossl_default_provider_init function assigns the addresses of ossl_default_get_params and ossl_default_gettable_params to the global variables c_get_params and c_gettable_params, respectively. This is where the data race occurs when multiple threads execute this function concurrently.

Analyzing Possible Solutions

  • Make the variables local: If these are made local they will be stored on the stack of the thread, thus each thread will have its own local copy, eliminating the race condition.
  • Mutex: A Mutex is a synchronization primitive that enforces exclusive access to shared resources. Only one thread can hold the lock at a time, and other threads must wait until the lock is released.
  • Double-checked locking: It reduces the overhead of acquiring a lock by first checking if the resource is already initialized. If it is, the lock is not acquired, and the thread can proceed without blocking.
  • Thread-Local Storage (TLS): Each thread has its own private copy of the variable. This eliminates the need for explicit synchronization mechanisms like mutexes, as each thread operates on its own isolated data.

Conclusion

The data race in ossl_default_provider_init is a serious issue that needs to be addressed. I hope this detailed explanation and analysis will help in finding a suitable solution.

Thanks for reading, and let's work together to make OpenSSL more robust and reliable!