diff --git a/ChangeLog.d/drbg-mutex.txt b/ChangeLog.d/drbg-mutex.txt
new file mode 100644
index 000000000..3ac5abfa8
--- /dev/null
+++ b/ChangeLog.d/drbg-mutex.txt
@@ -0,0 +1,5 @@
+Bugfix
+   * Fix a resource leak in CTR_DRBG and HMAC_DRBG when MBEDTLS_THREADING_C
+     is enabled, on platforms where initializing a mutex allocates resources.
+     This was a regression introduced in the previous release. Reported in
+     #4017, #4045 and #4071.
diff --git a/ChangeLog.d/rsa-mutex.txt b/ChangeLog.d/rsa-mutex.txt
new file mode 100644
index 000000000..2a477a9cb
--- /dev/null
+++ b/ChangeLog.d/rsa-mutex.txt
@@ -0,0 +1,13 @@
+Bugfix
+   * Ensure that calling mbedtls_rsa_free() or mbedtls_entropy_free()
+     twice is safe. This happens for RSA when some Mbed TLS library functions
+     fail. Such a double-free was not safe when MBEDTLS_THREADING_C was
+     enabled on platforms where freeing a mutex twice is not safe.
+   * Fix a resource leak in a bad-arguments case of mbedtls_rsa_gen_key()
+     when MBEDTLS_THREADING_C is enabled on platforms where initializing
+     a mutex allocates resources.
+
+Default behavior changes
+   * In mbedtls_rsa_context objects, the ver field was formerly documented
+     as always 0. It is now reserved for internal purposes and may take
+     different values.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 6b45021d0..fff4240c0 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1746,6 +1746,23 @@
  */
 //#define MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT
 
+/**
+ * \def MBEDTLS_TEST_HOOKS
+ *
+ * Enable features for invasive testing such as introspection functions and
+ * hooks for fault injection. This enables additional unit tests.
+ *
+ * Merely enabling this feature should not change the behavior of the product.
+ * It only adds new code, and new branching points where the default behavior
+ * is the same as when this feature is disabled.
+ * However, this feature increases the attack surface: there is an added
+ * risk of vulnerabilities, and more gadgets that can make exploits easier.
+ * Therefore this feature must never be enabled in production.
+ *
+ * Uncomment to enable invasive tests.
+ */
+//#define MBEDTLS_TEST_HOOKS
+
 /**
  * \def MBEDTLS_THREADING_ALT
  *
diff --git a/include/mbedtls/ctr_drbg.h b/include/mbedtls/ctr_drbg.h
index 278fbbbb7..6c099adf4 100644
--- a/include/mbedtls/ctr_drbg.h
+++ b/include/mbedtls/ctr_drbg.h
@@ -214,6 +214,13 @@ typedef struct mbedtls_ctr_drbg_context
     void *p_entropy;            /*!< The context for the entropy function. */
 
 #if defined(MBEDTLS_THREADING_C)
+    /* Invariant: the mutex is initialized if and only if f_entropy != NULL.
+     * This means that the mutex is initialized during the initial seeding
+     * in mbedtls_ctr_drbg_seed() and freed in mbedtls_ctr_drbg_free().
+     *
+     * Note that this invariant may change without notice. Do not rely on it
+     * and do not access the mutex directly in application code.
+     */
     mbedtls_threading_mutex_t mutex;
 #endif
 }
@@ -277,6 +284,15 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
  *                      device.
  */
 #endif
+#if defined(MBEDTLS_THREADING_C)
+/**
+ * \note                When Mbed TLS is built with threading support,
+ *                      after this function returns successfully,
+ *                      it is safe to call mbedtls_ctr_drbg_random()
+ *                      from multiple threads. Other operations, including
+ *                      reseeding, are not thread-safe.
+ */
+#endif /* MBEDTLS_THREADING_C */
 /**
  * \param ctx           The CTR_DRBG context to seed.
  *                      It must have been initialized with
@@ -286,6 +302,8 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
  *                      the same context unless you call
  *                      mbedtls_ctr_drbg_free() and mbedtls_ctr_drbg_init()
  *                      again first.
+ *                      After a failed call to mbedtls_ctr_drbg_seed(),
+ *                      you must call mbedtls_ctr_drbg_free().
  * \param f_entropy     The entropy callback, taking as arguments the
  *                      \p p_entropy context, the buffer to fill, and the
  *                      length of the buffer.
@@ -377,6 +395,11 @@ void mbedtls_ctr_drbg_set_reseed_interval( mbedtls_ctr_drbg_context *ctx,
  * \brief               This function reseeds the CTR_DRBG context, that is
  *                      extracts data from the entropy source.
  *
+ * \note                This function is not thread-safe. It is not safe
+ *                      to call this function if another thread might be
+ *                      concurrently obtaining random numbers from the same
+ *                      context or updating or reseeding the same context.
+ *
  * \param ctx           The CTR_DRBG context.
  * \param additional    Additional data to add to the state. Can be \c NULL.
  * \param len           The length of the additional data.
@@ -394,6 +417,11 @@ int mbedtls_ctr_drbg_reseed( mbedtls_ctr_drbg_context *ctx,
 /**
  * \brief              This function updates the state of the CTR_DRBG context.
  *
+ * \note               This function is not thread-safe. It is not safe
+ *                     to call this function if another thread might be
+ *                     concurrently obtaining random numbers from the same
+ *                     context or updating or reseeding the same context.
+ *
  * \param ctx          The CTR_DRBG context.
  * \param additional   The data to update the state with. This must not be
  *                     \c NULL unless \p add_len is \c 0.
@@ -417,6 +445,11 @@ int mbedtls_ctr_drbg_update_ret( mbedtls_ctr_drbg_context *ctx,
  * This function automatically reseeds if the reseed counter is exceeded
  * or prediction resistance is enabled.
  *
+ * \note                This function is not thread-safe. It is not safe
+ *                      to call this function if another thread might be
+ *                      concurrently obtaining random numbers from the same
+ *                      context or updating or reseeding the same context.
+ *
  * \param p_rng         The CTR_DRBG context. This must be a pointer to a
  *                      #mbedtls_ctr_drbg_context structure.
  * \param output        The buffer to fill.
@@ -445,8 +478,16 @@ int mbedtls_ctr_drbg_random_with_add( void *p_rng,
  *
  * This function automatically reseeds if the reseed counter is exceeded
  * or prediction resistance is enabled.
- *
- *
+ */
+#if defined(MBEDTLS_THREADING_C)
+/**
+ * \note                When Mbed TLS is built with threading support,
+ *                      it is safe to call mbedtls_ctr_drbg_random()
+ *                      from multiple threads. Other operations, including
+ *                      reseeding, are not thread-safe.
+ */
+#endif /* MBEDTLS_THREADING_C */
+/**
  * \param p_rng         The CTR_DRBG context. This must be a pointer to a
  *                      #mbedtls_ctr_drbg_context structure.
  * \param output        The buffer to fill.
diff --git a/include/mbedtls/entropy.h b/include/mbedtls/entropy.h
index 1e1d3f56e..1d6e9b821 100644
--- a/include/mbedtls/entropy.h
+++ b/include/mbedtls/entropy.h
@@ -147,13 +147,15 @@ mbedtls_entropy_source_state;
  */
 typedef struct mbedtls_entropy_context
 {
-    int accumulator_started;
+    int accumulator_started; /* 0 after init.
+                              * 1 after the first update.
+                              * -1 after free. */
 #if defined(MBEDTLS_ENTROPY_SHA512_ACCUMULATOR)
     mbedtls_sha512_context  accumulator;
 #else
     mbedtls_sha256_context  accumulator;
 #endif
-    int             source_count;
+    int             source_count; /* Number of entries used in source. */
     mbedtls_entropy_source_state    source[MBEDTLS_ENTROPY_MAX_SOURCES];
 #if defined(MBEDTLS_HAVEGE_C)
     mbedtls_havege_state    havege_data;
diff --git a/include/mbedtls/hmac_drbg.h b/include/mbedtls/hmac_drbg.h
index 970c033c1..5718e187a 100644
--- a/include/mbedtls/hmac_drbg.h
+++ b/include/mbedtls/hmac_drbg.h
@@ -128,6 +128,14 @@ typedef struct mbedtls_hmac_drbg_context
     void *p_entropy;            /*!< context for the entropy function        */
 
 #if defined(MBEDTLS_THREADING_C)
+    /* Invariant: the mutex is initialized if and only if
+     * md_ctx->md_info != NULL. This means that the mutex is initialized
+     * during the initial seeding in mbedtls_hmac_drbg_seed() or
+     * mbedtls_hmac_drbg_seed_buf() and freed in mbedtls_ctr_drbg_free().
+     *
+     * Note that this invariant may change without notice. Do not rely on it
+     * and do not access the mutex directly in application code.
+     */
     mbedtls_threading_mutex_t mutex;
 #endif
 } mbedtls_hmac_drbg_context;
@@ -177,7 +185,17 @@ void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx );
  * \note                During the initial seeding, this function calls
  *                      the entropy source to obtain a nonce
  *                      whose length is half the entropy length.
- *
+ */
+#if defined(MBEDTLS_THREADING_C)
+/**
+ * \note                When Mbed TLS is built with threading support,
+ *                      after this function returns successfully,
+ *                      it is safe to call mbedtls_hmac_drbg_random()
+ *                      from multiple threads. Other operations, including
+ *                      reseeding, are not thread-safe.
+ */
+#endif /* MBEDTLS_THREADING_C */
+/**
  * \param ctx           HMAC_DRBG context to be seeded.
  * \param md_info       MD algorithm to use for HMAC_DRBG.
  * \param f_entropy     The entropy callback, taking as arguments the
@@ -216,7 +234,17 @@ int mbedtls_hmac_drbg_seed( mbedtls_hmac_drbg_context *ctx,
  *
  * This function is meant for use in algorithms that need a pseudorandom
  * input such as deterministic ECDSA.
- *
+ */
+#if defined(MBEDTLS_THREADING_C)
+/**
+ * \note                When Mbed TLS is built with threading support,
+ *                      after this function returns successfully,
+ *                      it is safe to call mbedtls_hmac_drbg_random()
+ *                      from multiple threads. Other operations, including
+ *                      reseeding, are not thread-safe.
+ */
+#endif /* MBEDTLS_THREADING_C */
+/**
  * \param ctx           HMAC_DRBG context to be initialised.
  * \param md_info       MD algorithm to use for HMAC_DRBG.
  * \param data          Concatenation of the initial entropy string and
@@ -279,6 +307,11 @@ void mbedtls_hmac_drbg_set_reseed_interval( mbedtls_hmac_drbg_context *ctx,
 /**
  * \brief               This function updates the state of the HMAC_DRBG context.
  *
+ * \note                This function is not thread-safe. It is not safe
+ *                      to call this function if another thread might be
+ *                      concurrently obtaining random numbers from the same
+ *                      context or updating or reseeding the same context.
+ *
  * \param ctx           The HMAC_DRBG context.
  * \param additional    The data to update the state with.
  *                      If this is \c NULL, there is no additional data.
@@ -295,6 +328,11 @@ int mbedtls_hmac_drbg_update_ret( mbedtls_hmac_drbg_context *ctx,
  * \brief               This function reseeds the HMAC_DRBG context, that is
  *                      extracts data from the entropy source.
  *
+ * \note                This function is not thread-safe. It is not safe
+ *                      to call this function if another thread might be
+ *                      concurrently obtaining random numbers from the same
+ *                      context or updating or reseeding the same context.
+ *
  * \param ctx           The HMAC_DRBG context.
  * \param additional    Additional data to add to the state.
  *                      If this is \c NULL, there is no additional data
@@ -320,6 +358,11 @@ int mbedtls_hmac_drbg_reseed( mbedtls_hmac_drbg_context *ctx,
  * This function automatically reseeds if the reseed counter is exceeded
  * or prediction resistance is enabled.
  *
+ * \note                This function is not thread-safe. It is not safe
+ *                      to call this function if another thread might be
+ *                      concurrently obtaining random numbers from the same
+ *                      context or updating or reseeding the same context.
+ *
  * \param p_rng         The HMAC_DRBG context. This must be a pointer to a
  *                      #mbedtls_hmac_drbg_context structure.
  * \param output        The buffer to fill.
@@ -349,7 +392,16 @@ int mbedtls_hmac_drbg_random_with_add( void *p_rng,
  *
  * This function automatically reseeds if the reseed counter is exceeded
  * or prediction resistance is enabled.
- *
+ */
+#if defined(MBEDTLS_THREADING_C)
+/**
+ * \note                When Mbed TLS is built with threading support,
+ *                      it is safe to call mbedtls_ctr_drbg_random()
+ *                      from multiple threads. Other operations, including
+ *                      reseeding, are not thread-safe.
+ */
+#endif /* MBEDTLS_THREADING_C */
+/**
  * \param p_rng         The HMAC_DRBG context. This must be a pointer to a
  *                      #mbedtls_hmac_drbg_context structure.
  * \param output        The buffer to fill.
diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index 188c37cf3..b2f65334f 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -124,7 +124,10 @@ extern "C" {
  */
 typedef struct mbedtls_rsa_context
 {
-    int ver;                    /*!<  Always 0.*/
+    int ver;                    /*!<  Reserved for internal purposes.
+                                 *    Do not set this field in application
+                                 *    code. Its meaning might change without
+                                 *    notice. */
     size_t len;                 /*!<  The size of \p N in Bytes. */
 
     mbedtls_mpi N;              /*!<  The public modulus. */
@@ -154,6 +157,7 @@ typedef struct mbedtls_rsa_context
                                      mask generating function used in the
                                      EME-OAEP and EMSA-PSS encodings. */
 #if defined(MBEDTLS_THREADING_C)
+    /* Invariant: the mutex is initialized iff ver != 0. */
     mbedtls_threading_mutex_t mutex;    /*!<  Thread-safety mutex. */
 #endif
 }
diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index a8183a6ef..45161ce46 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -73,6 +73,9 @@ extern "C" {
 typedef struct mbedtls_threading_mutex_t
 {
     pthread_mutex_t mutex;
+    /* is_valid is 0 after a failed init or a free, and nonzero after a
+     * successful init. This field is not considered part of the public
+     * API of Mbed TLS and may change without notice. */
     char is_valid;
 } mbedtls_threading_mutex_t;
 #endif
diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index e92008bbe..90264e844 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -83,10 +83,6 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx )
     memset( ctx, 0, sizeof( mbedtls_ctr_drbg_context ) );
 
     ctx->reseed_interval = MBEDTLS_CTR_DRBG_RESEED_INTERVAL;
-
-#if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_init( &ctx->mutex );
-#endif
 }
 
 /*
@@ -99,14 +95,13 @@ void mbedtls_ctr_drbg_free( mbedtls_ctr_drbg_context *ctx )
         return;
 
 #if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_free( &ctx->mutex );
+    /* The mutex is initialized iff f_entropy is set. */
+    if( ctx->f_entropy != NULL )
+        mbedtls_mutex_free( &ctx->mutex );
 #endif
     mbedtls_aes_free( &ctx->aes_ctx );
     mbedtls_platform_zeroize( ctx, sizeof( mbedtls_ctr_drbg_context ) );
     ctx->reseed_interval = MBEDTLS_CTR_DRBG_RESEED_INTERVAL;
-#if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_init( &ctx->mutex );
-#endif
 }
 
 void mbedtls_ctr_drbg_set_prediction_resistance( mbedtls_ctr_drbg_context *ctx, int resistance )
@@ -422,6 +417,11 @@ int mbedtls_ctr_drbg_seed( mbedtls_ctr_drbg_context *ctx,
 
     memset( key, 0, MBEDTLS_CTR_DRBG_KEYSIZE );
 
+    /* The mutex is initialized iff f_entropy is set. */
+#if defined(MBEDTLS_THREADING_C)
+    mbedtls_mutex_init( &ctx->mutex );
+#endif
+
     mbedtls_aes_init( &ctx->aes_ctx );
 
     ctx->f_entropy = f_entropy;
diff --git a/library/entropy.c b/library/entropy.c
index 666c55654..c5f414a01 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -146,6 +146,11 @@ void mbedtls_entropy_init( mbedtls_entropy_context *ctx )
 
 void mbedtls_entropy_free( mbedtls_entropy_context *ctx )
 {
+    /* If the context was already free, don't call free() again.
+     * This is important for mutexes which don't allow double-free. */
+    if( ctx->accumulator_started == -1 )
+        return;
+
 #if defined(MBEDTLS_HAVEGE_C)
     mbedtls_havege_free( &ctx->havege_data );
 #endif
@@ -162,7 +167,7 @@ void mbedtls_entropy_free( mbedtls_entropy_context *ctx )
 #endif
     ctx->source_count = 0;
     mbedtls_platform_zeroize( ctx->source, sizeof( ctx->source ) );
-    ctx->accumulator_started = 0;
+    ctx->accumulator_started = -1;
 }
 
 int mbedtls_entropy_add_source( mbedtls_entropy_context *ctx,
diff --git a/library/hmac_drbg.c b/library/hmac_drbg.c
index 10cbd462b..b45d61616 100644
--- a/library/hmac_drbg.c
+++ b/library/hmac_drbg.c
@@ -84,10 +84,6 @@ void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx )
     memset( ctx, 0, sizeof( mbedtls_hmac_drbg_context ) );
 
     ctx->reseed_interval = MBEDTLS_HMAC_DRBG_RESEED_INTERVAL;
-
-#if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_init( &ctx->mutex );
-#endif
 }
 
 /*
@@ -159,6 +155,10 @@ int mbedtls_hmac_drbg_seed_buf( mbedtls_hmac_drbg_context *ctx,
     if( ( ret = mbedtls_md_setup( &ctx->md_ctx, md_info, 1 ) ) != 0 )
         return( ret );
 
+#if defined(MBEDTLS_THREADING_C)
+    mbedtls_mutex_init( &ctx->mutex );
+#endif
+
     /*
      * Set initial working state.
      * Use the V memory location, which is currently all 0, to initialize the
@@ -284,6 +284,11 @@ int mbedtls_hmac_drbg_seed( mbedtls_hmac_drbg_context *ctx,
     if( ( ret = mbedtls_md_setup( &ctx->md_ctx, md_info, 1 ) ) != 0 )
         return( ret );
 
+    /* The mutex is initialized iff the md context is set up. */
+#if defined(MBEDTLS_THREADING_C)
+    mbedtls_mutex_init( &ctx->mutex );
+#endif
+
     md_size = mbedtls_md_get_size( md_info );
 
     /*
@@ -451,14 +456,13 @@ void mbedtls_hmac_drbg_free( mbedtls_hmac_drbg_context *ctx )
         return;
 
 #if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_free( &ctx->mutex );
+    /* The mutex is initialized iff the md context is set up. */
+    if( ctx->md_ctx.md_info != NULL )
+        mbedtls_mutex_free( &ctx->mutex );
 #endif
     mbedtls_md_free( &ctx->md_ctx );
     mbedtls_platform_zeroize( ctx, sizeof( mbedtls_hmac_drbg_context ) );
     ctx->reseed_interval = MBEDTLS_HMAC_DRBG_RESEED_INTERVAL;
-#if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_init( &ctx->mutex );
-#endif
 }
 
 #if defined(MBEDTLS_FS_IO)
diff --git a/library/rsa.c b/library/rsa.c
index 000754649..c8c23dba8 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -520,6 +520,9 @@ void mbedtls_rsa_init( mbedtls_rsa_context *ctx,
     mbedtls_rsa_set_padding( ctx, padding, hash_id );
 
 #if defined(MBEDTLS_THREADING_C)
+    /* Set ctx->ver to nonzero to indicate that the mutex has been
+     * initialized and will need to be freed. */
+    ctx->ver = 1;
     mbedtls_mutex_init( &ctx->mutex );
 #endif
 }
@@ -567,9 +570,6 @@ int mbedtls_rsa_gen_key( mbedtls_rsa_context *ctx,
     RSA_VALIDATE_RET( ctx != NULL );
     RSA_VALIDATE_RET( f_rng != NULL );
 
-    if( nbits < 128 || exponent < 3 || nbits % 2 != 0 )
-        return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
-
     /*
      * If the modulus is 1024 bit long or shorter, then the security strength of
      * the RSA algorithm is less than or equal to 80 bits and therefore an error
@@ -582,6 +582,12 @@ int mbedtls_rsa_gen_key( mbedtls_rsa_context *ctx,
     mbedtls_mpi_init( &G );
     mbedtls_mpi_init( &L );
 
+    if( nbits < 128 || exponent < 3 || nbits % 2 != 0 )
+    {
+        ret = MBEDTLS_ERR_RSA_BAD_INPUT_DATA;
+        goto cleanup;
+    }
+
     /*
      * find primes P and Q with Q < P so that:
      * 1.  |P-Q| > 2^( nbits / 2 - 100 )
@@ -659,7 +665,9 @@ cleanup:
     if( ret != 0 )
     {
         mbedtls_rsa_free( ctx );
-        return( MBEDTLS_ERR_RSA_KEY_GEN_FAILED + ret );
+        if( ( -ret & ~0x7f ) == 0 )
+            ret = MBEDTLS_ERR_RSA_KEY_GEN_FAILED + ret;
+        return( ret );
     }
 
     return( 0 );
@@ -2502,7 +2510,6 @@ int mbedtls_rsa_copy( mbedtls_rsa_context *dst, const mbedtls_rsa_context *src )
     RSA_VALIDATE_RET( dst != NULL );
     RSA_VALIDATE_RET( src != NULL );
 
-    dst->ver = src->ver;
     dst->len = src->len;
 
     MBEDTLS_MPI_CHK( mbedtls_mpi_copy( &dst->N, &src->N ) );
@@ -2561,7 +2568,12 @@ void mbedtls_rsa_free( mbedtls_rsa_context *ctx )
 #endif /* MBEDTLS_RSA_NO_CRT */
 
 #if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_free( &ctx->mutex );
+    /* Free the mutex, but only if it hasn't been freed already. */
+    if( ctx->ver != 0 )
+    {
+        mbedtls_mutex_free( &ctx->mutex );
+        ctx->ver = 0;
+    }
 #endif
 }
 
diff --git a/library/threading.c b/library/threading.c
index f4f29cff5..0dc5488c1 100644
--- a/library/threading.c
+++ b/library/threading.c
@@ -98,6 +98,12 @@ static void threading_mutex_init_pthread( mbedtls_threading_mutex_t *mutex )
     if( mutex == NULL )
         return;
 
+    /* A nonzero value of is_valid indicates a successfully initialized
+     * mutex. This is a workaround for not being able to return an error
+     * code for this function. The lock/unlock functions return an error
+     * if is_valid is nonzero. The Mbed TLS unit test code uses this field
+     * to distinguish more states of the mutex; see helpers.function for
+     * details. */
     mutex->is_valid = pthread_mutex_init( &mutex->mutex, NULL ) == 0;
 }
 
diff --git a/library/version_features.c b/library/version_features.c
index cbf38dc2c..8c8e815e9 100644
--- a/library/version_features.c
+++ b/library/version_features.c
@@ -553,6 +553,9 @@ static const char *features[] = {
 #if defined(MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT)
     "MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT",
 #endif /* MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT */
+#if defined(MBEDTLS_TEST_HOOKS)
+    "MBEDTLS_TEST_HOOKS",
+#endif /* MBEDTLS_TEST_HOOKS */
 #if defined(MBEDTLS_THREADING_ALT)
     "MBEDTLS_THREADING_ALT",
 #endif /* MBEDTLS_THREADING_ALT */
diff --git a/programs/ssl/query_config.c b/programs/ssl/query_config.c
index 798c9178f..0a7eb1aec 100644
--- a/programs/ssl/query_config.c
+++ b/programs/ssl/query_config.c
@@ -1475,6 +1475,14 @@ int query_config( const char *config )
     }
 #endif /* MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT */
 
+#if defined(MBEDTLS_TEST_HOOKS)
+    if( strcmp( "MBEDTLS_TEST_HOOKS", config ) == 0 )
+    {
+        MACRO_EXPANSION_TO_STR( MBEDTLS_TEST_HOOKS );
+        return( 0 );
+    }
+#endif /* MBEDTLS_TEST_HOOKS */
+
 #if defined(MBEDTLS_THREADING_ALT)
     if( strcmp( "MBEDTLS_THREADING_ALT", config ) == 0 )
     {
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 9ce1377b2..1e56c3e62 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -1324,7 +1324,7 @@ component_test_malloc_0_null () {
     msg "build: malloc(0) returns NULL (ASan+UBSan build)"
     scripts/config.pl full
     scripts/config.pl unset MBEDTLS_MEMORY_BUFFER_ALLOC_C
-    make CC=gcc CFLAGS="'-DMBEDTLS_CONFIG_FILE=\"$PWD/tests/configs/config-wrapper-malloc-0-null.h\"' -O -Werror -Wall -Wextra -fsanitize=address,undefined" LDFLAGS='-fsanitize=address,undefined'
+    make CC=gcc CFLAGS="'-DMBEDTLS_CONFIG_FILE=\"$PWD/tests/configs/config-wrapper-malloc-0-null.h\"' -O $ASAN_CFLAGS" LDFLAGS="$ASAN_CFLAGS"
 
     msg "test: malloc(0) returns NULL (ASan+UBSan build)"
     make test
diff --git a/tests/suites/helpers.function b/tests/suites/helpers.function
index 9403d9905..ceac6703f 100644
--- a/tests/suites/helpers.function
+++ b/tests/suites/helpers.function
@@ -46,6 +46,12 @@ typedef UINT32 uint32_t;
 #include <strings.h>
 #endif
 
+#if defined(MBEDTLS_THREADING_C) && defined(MBEDTLS_THREADING_PTHREAD) && \
+    defined(MBEDTLS_TEST_HOOKS)
+#include "mbedtls/threading.h"
+#define MBEDTLS_TEST_MUTEX_USAGE
+#endif
+
 /*
  * Define the two macros
  *
@@ -371,6 +377,9 @@ static struct
     const char *test;
     const char *filename;
     int line_no;
+#if defined(MBEDTLS_TEST_MUTEX_USAGE)
+    const char *mutex_usage_error;
+#endif
 }
 test_info;
 
@@ -777,3 +786,202 @@ int mbedtls_test_hexcmp( uint8_t * a, uint8_t * b, uint32_t a_len, uint32_t b_le
     }
     return ret;
 }
+
+#if defined(MBEDTLS_TEST_MUTEX_USAGE)
+/** Mutex usage verification framework.
+ *
+ * The mutex usage verification code below aims to detect bad usage of
+ * Mbed TLS's mutex abstraction layer at runtime. Note that this is solely
+ * about the use of the mutex itself, not about checking whether the mutex
+ * correctly protects whatever it is supposed to protect.
+ *
+ * The normal usage of a mutex is:
+ * ```
+ * digraph mutex_states {
+ *   "UNINITIALIZED"; // the initial state
+ *   "IDLE";
+ *   "FREED";
+ *   "LOCKED";
+ *   "UNINITIALIZED" -> "IDLE" [label="init"];
+ *   "FREED" -> "IDLE" [label="init"];
+ *   "IDLE" -> "LOCKED" [label="lock"];
+ *   "LOCKED" -> "IDLE" [label="unlock"];
+ *   "IDLE" -> "FREED" [label="free"];
+ * }
+ * ```
+ *
+ * All bad transitions that can be unambiguously detected are reported.
+ * An attempt to use an uninitialized mutex cannot be detected in general
+ * since the memory content may happen to denote a valid state. For the same
+ * reason, a double init cannot be detected.
+ * All-bits-zero is the state of a freed mutex, which is distinct from an
+ * initialized mutex, so attempting to use zero-initialized memory as a mutex
+ * without calling the init function is detected.
+ *
+ * The framework attempts to detect missing calls to init and free by counting
+ * calls to init and free. If there are more calls to init than free, this
+ * means that a mutex is not being freed somewhere, which is a memory leak
+ * on platforms where a mutex consumes resources other than the
+ * mbedtls_threading_mutex_t object itself. If there are more calls to free
+ * than init, this indicates a missing init, which is likely to be detected
+ * by an attempt to lock the mutex as well. A limitation of this framework is
+ * that it cannot detect scenarios where there is exactly the same number of
+ * calls to init and free but the calls don't match. A bug like this is
+ * unlikely to happen uniformly throughout the whole test suite though.
+ *
+ * If an error is detected, this framework will report what happened and the
+ * test case will be marked as failed. Unfortunately, the error report cannot
+ * indicate the exact location of the problematic call. To locate the error,
+ * use a debugger and set a breakpoint on mbedtls_test_mutex_usage_error().
+ */
+enum value_of_mutex_is_valid_field
+{
+    /* Potential values for the is_valid field of mbedtls_threading_mutex_t.
+     * Note that MUTEX_FREED must be 0 and MUTEX_IDLE must be 1 for
+     * compatibility with threading_mutex_init_pthread() and
+     * threading_mutex_free_pthread(). MUTEX_LOCKED could be any nonzero
+     * value. */
+    MUTEX_FREED = 0, //!< Set by threading_mutex_free_pthread
+    MUTEX_IDLE = 1, //!< Set by threading_mutex_init_pthread and by our unlock
+    MUTEX_LOCKED = 2, //!< Set by our lock
+};
+
+typedef struct
+{
+    void (*init)( mbedtls_threading_mutex_t * );
+    void (*free)( mbedtls_threading_mutex_t * );
+    int (*lock)( mbedtls_threading_mutex_t * );
+    int (*unlock)( mbedtls_threading_mutex_t * );
+} mutex_functions_t;
+static mutex_functions_t mutex_functions;
+
+/** The total number of calls to mbedtls_mutex_init(), minus the total number
+ * of calls to mbedtls_mutex_free().
+ *
+ * Reset to 0 after each test case.
+ */
+static int live_mutexes;
+
+static void mbedtls_test_mutex_usage_error( mbedtls_threading_mutex_t *mutex,
+                                            const char *msg )
+{
+    (void) mutex;
+    if( test_info.mutex_usage_error == NULL )
+        test_info.mutex_usage_error = msg;
+    mbedtls_fprintf( stdout, "[mutex: %s] ", msg );
+    /* Don't mark the test as failed yet. This way, if the test fails later
+     * for a functional reason, the test framework will report the message
+     * and location for this functional reason. If the test passes,
+     * mbedtls_test_mutex_usage_check() will mark it as failed. */
+}
+
+static void mbedtls_test_wrap_mutex_init( mbedtls_threading_mutex_t *mutex )
+{
+    mutex_functions.init( mutex );
+    if( mutex->is_valid )
+        ++live_mutexes;
+}
+
+static void mbedtls_test_wrap_mutex_free( mbedtls_threading_mutex_t *mutex )
+{
+    switch( mutex->is_valid )
+    {
+        case MUTEX_FREED:
+            mbedtls_test_mutex_usage_error( mutex, "free without init or double free" );
+            break;
+        case MUTEX_IDLE:
+            /* Do nothing. The underlying free function will reset is_valid
+             * to 0. */
+            break;
+        case MUTEX_LOCKED:
+            mbedtls_test_mutex_usage_error( mutex, "free without unlock" );
+            break;
+        default:
+            mbedtls_test_mutex_usage_error( mutex, "corrupted state" );
+            break;
+    }
+    if( mutex->is_valid )
+        --live_mutexes;
+    mutex_functions.free( mutex );
+}
+
+static int mbedtls_test_wrap_mutex_lock( mbedtls_threading_mutex_t *mutex )
+{
+    int ret = mutex_functions.lock( mutex );
+    switch( mutex->is_valid )
+    {
+        case MUTEX_FREED:
+            mbedtls_test_mutex_usage_error( mutex, "lock without init" );
+            break;
+        case MUTEX_IDLE:
+            if( ret == 0 )
+                mutex->is_valid = 2;
+            break;
+        case MUTEX_LOCKED:
+            mbedtls_test_mutex_usage_error( mutex, "double lock" );
+            break;
+        default:
+            mbedtls_test_mutex_usage_error( mutex, "corrupted state" );
+            break;
+    }
+    return( ret );
+}
+
+static int mbedtls_test_wrap_mutex_unlock( mbedtls_threading_mutex_t *mutex )
+{
+    int ret = mutex_functions.unlock( mutex );
+    switch( mutex->is_valid )
+    {
+        case MUTEX_FREED:
+            mbedtls_test_mutex_usage_error( mutex, "unlock without init" );
+            break;
+        case MUTEX_IDLE:
+            mbedtls_test_mutex_usage_error( mutex, "unlock without lock" );
+            break;
+        case MUTEX_LOCKED:
+            if( ret == 0 )
+                mutex->is_valid = MUTEX_IDLE;
+            break;
+        default:
+            mbedtls_test_mutex_usage_error( mutex, "corrupted state" );
+            break;
+    }
+    return( ret );
+}
+
+static void mbedtls_test_mutex_usage_init( void )
+{
+    mutex_functions.init = mbedtls_mutex_init;
+    mutex_functions.free = mbedtls_mutex_free;
+    mutex_functions.lock = mbedtls_mutex_lock;
+    mutex_functions.unlock = mbedtls_mutex_unlock;
+    mbedtls_mutex_init = &mbedtls_test_wrap_mutex_init;
+    mbedtls_mutex_free = &mbedtls_test_wrap_mutex_free;
+    mbedtls_mutex_lock = &mbedtls_test_wrap_mutex_lock;
+    mbedtls_mutex_unlock = &mbedtls_test_wrap_mutex_unlock;
+}
+
+static void mbedtls_test_mutex_usage_check( void )
+{
+    if( live_mutexes != 0 )
+    {
+        /* A positive number (more init than free) means that a mutex resource
+         * is leaking (on platforms where a mutex consumes more than the
+         * mbedtls_threading_mutex_t object itself). The rare case of a
+         * negative number means a missing init somewhere. */
+        mbedtls_fprintf( stdout, "[mutex: %d leaked] ", live_mutexes );
+        live_mutexes = 0;
+        if( test_info.mutex_usage_error == NULL )
+            test_info.mutex_usage_error = "missing free";
+    }
+    if( test_info.mutex_usage_error != NULL &&
+        test_info.result != TEST_RESULT_FAILED )
+    {
+        /* Functionally, the test passed. But there was a mutex usage error,
+         * so mark the test as failed after all. */
+        test_fail( "Mutex usage error", __LINE__, __FILE__ );
+    }
+    test_info.mutex_usage_error = NULL;
+}
+
+#endif /* MBEDTLS_TEST_MUTEX_USAGE */
diff --git a/tests/suites/host_test.function b/tests/suites/host_test.function
index 1178135c5..3f8a945ea 100644
--- a/tests/suites/host_test.function
+++ b/tests/suites/host_test.function
@@ -412,6 +412,10 @@ int execute_tests( int argc , const char ** argv )
     mbedtls_memory_buffer_alloc_init( alloc_buf, sizeof( alloc_buf ) );
 #endif
 
+#if defined(MBEDTLS_TEST_MUTEX_USAGE)
+    mbedtls_test_mutex_usage_init( );
+#endif
+
     /*
      * The C standard doesn't guarantee that all-bits-0 is the representation
      * of a NULL pointer. We do however use that in our code for initializing
diff --git a/tests/suites/main_test.function b/tests/suites/main_test.function
index 9f366907b..2aecf8142 100644
--- a/tests/suites/main_test.function
+++ b/tests/suites/main_test.function
@@ -176,6 +176,10 @@ void execute_function_ptr(TestWrapper_t fp, void **params)
 #else
     fp( params );
 #endif
+
+#if defined(MBEDTLS_TEST_MUTEX_USAGE)
+    mbedtls_test_mutex_usage_check( );
+#endif /* MBEDTLS_TEST_MUTEX_USAGE */
 }
 
 /**
diff --git a/tests/suites/test_suite_entropy.data b/tests/suites/test_suite_entropy.data
index 5cff39984..8ad8760e2 100644
--- a/tests/suites/test_suite_entropy.data
+++ b/tests/suites/test_suite_entropy.data
@@ -1,3 +1,9 @@
+Entropy init-free-free
+entropy_init_free:0
+
+Entropy init-free-init-free
+entropy_init_free:1
+
 Create NV seed_file
 nv_seed_file_create:
 
diff --git a/tests/suites/test_suite_entropy.function b/tests/suites/test_suite_entropy.function
index 889d3bca7..f4f9693fb 100644
--- a/tests/suites/test_suite_entropy.function
+++ b/tests/suites/test_suite_entropy.function
@@ -125,6 +125,28 @@ int read_nv_seed( unsigned char *buf, size_t buf_len )
  * END_DEPENDENCIES
  */
 
+/* BEGIN_CASE */
+void entropy_init_free( int reinit )
+{
+    mbedtls_entropy_context ctx;
+
+    /* Double free is not explicitly documented to work, but it is convenient
+     * to call mbedtls_entropy_free() unconditionally on an error path without
+     * checking whether it has already been called in the success path. */
+
+    mbedtls_entropy_init( &ctx );
+    mbedtls_entropy_free( &ctx );
+
+    if( reinit )
+        mbedtls_entropy_init( &ctx );
+    mbedtls_entropy_free( &ctx );
+
+    /* This test case always succeeds, functionally speaking. A plausible
+     * bug might trigger an invalid pointer dereference or a memory leak. */
+    goto exit;
+}
+/* END_CASE */
+
 /* BEGIN_CASE depends_on:MBEDTLS_ENTROPY_NV_SEED:MBEDTLS_FS_IO */
 void entropy_seed_file( char * path, int ret )
 {
@@ -191,6 +213,9 @@ void entropy_func_len( int len, int ret )
 
     for( j = len; j < sizeof( buf ); j++ )
         TEST_ASSERT( acc[j] == 0 );
+
+exit:
+    mbedtls_entropy_free( &ctx );
 }
 /* END_CASE */
 
diff --git a/tests/suites/test_suite_rsa.data b/tests/suites/test_suite_rsa.data
index 5e4de17e2..4ed833903 100644
--- a/tests/suites/test_suite_rsa.data
+++ b/tests/suites/test_suite_rsa.data
@@ -1,6 +1,12 @@
 RSA parameter validation
 rsa_invalid_param:
 
+RSA init-free-free
+rsa_init_free:0
+
+RSA init-free-init-free
+rsa_init_free:1
+
 RSA PKCS1 Verify v1.5 CAVS #1
 depends_on:MBEDTLS_SHA1_C:MBEDTLS_PKCS1_V15
 # Good padding but wrong hash
diff --git a/tests/suites/test_suite_rsa.function b/tests/suites/test_suite_rsa.function
index f8a2dad0d..89b419fbd 100644
--- a/tests/suites/test_suite_rsa.function
+++ b/tests/suites/test_suite_rsa.function
@@ -465,6 +465,29 @@ exit:
 }
 /* END_CASE */
 
+/* BEGIN_CASE */
+void rsa_init_free( int reinit )
+{
+    mbedtls_rsa_context ctx;
+
+    /* Double free is not explicitly documented to work, but we rely on it
+     * even inside the library so that you can call mbedtls_rsa_free()
+     * unconditionally on an error path without checking whether it has
+     * already been called in the success path. */
+
+    mbedtls_rsa_init( &ctx, 0, 0 );
+    mbedtls_rsa_free( &ctx );
+
+    if( reinit )
+        mbedtls_rsa_init( &ctx, 0, 0 );
+    mbedtls_rsa_free( &ctx );
+
+    /* This test case always succeeds, functionally speaking. A plausible
+     * bug might trigger an invalid pointer dereference or a memory leak. */
+    goto exit;
+}
+/* END_CASE */
+
 /* BEGIN_CASE */
 void mbedtls_rsa_pkcs1_sign( data_t * message_str, int padding_mode,
                              int digest, int mod, int radix_P, char * input_P,