[tools] Surround mbedTLS's CTR_DRBG operations with spinlocks

mbedTLS explicitly says that CTR_DRBG operations are not thread-safe.
Previously, our Protected Files utility code (currently used only for
the `gramine-sgx-pf-crypt` tool) was rendered thread-unsafe because it
used CTR_DRBG without any synchronization (note that we do not build
mbedTLS with `MBEDTLS_THREADING_C`). This was not a security issue in
Gramine because the only user (`gramine-sgx-pf-crypt`) is
single-threaded, but it is a potential vulnerability if this utility
code is reused in some other, multi-threaded scenarios.

To make CTR_DRBG thread-safe, this commit surrounds the relevant logic
with spinlocks. The only identified mbedTLS function that needs this is
`mbedtls_ctr_drbg_random()`. See also mbedTLS docs.

Kudos to Petr Evstifeev (Villain88) who reported this issue.

Signed-off-by: Dmitrii Kuvaiskii <[email protected]>

tools/sgx/common/pf_util.c

@@ -25,6 +25,7 @@
 #include "api.h"
 #include "path_utils.h"
 #include "perm.h"
+#include "spinlock.h"
 #include "util.h"
 
 /* High-level protected files helper functions. */
@@ -185,8 +186,14 @@ pf_status_t mbedtls_aes_gcm_decrypt(const pf_key_t* key, const pf_iv_t* iv, cons
 static mbedtls_entropy_context g_entropy;
 static mbedtls_ctr_drbg_context g_prng;
 
+/* CTR_DRBG functions of mbedTLS are not thread-safe, must explicitly sync them */
+static spinlock_t g_mbedtls_ctr_drbg_lock = INIT_SPINLOCK_UNLOCKED;
+
 static pf_status_t mbedtls_random(uint8_t* buffer, size_t size) {
-    if (mbedtls_ctr_drbg_random(&g_prng, buffer, size) != 0) {
+    spinlock_lock(&g_mbedtls_ctr_drbg_lock);
+    int ret = mbedtls_ctr_drbg_random(&g_prng, buffer, size);
+    spinlock_unlock(&g_mbedtls_ctr_drbg_lock);
+    if (ret != 0) {
         ERROR("Failed to get random bytes\n");
         return PF_STATUS_CALLBACK_FAILED;
     }
@@ -226,7 +233,9 @@ int pf_init(void) {
 int pf_generate_wrap_key(const char* wrap_key_path) {
     pf_key_t wrap_key;
 
+    spinlock_lock(&g_mbedtls_ctr_drbg_lock);
     int ret = mbedtls_ctr_drbg_random(&g_prng, (unsigned char*)&wrap_key, sizeof(wrap_key));
+    spinlock_unlock(&g_mbedtls_ctr_drbg_lock);
     if (ret != 0) {
         ERROR("Failed to read random bytes: %d\n", ret);
         return ret;