CORE: make sure activated fallback providers stay activated

Calling 'OSSL_PROVIDER_available(NULL, "default")' would search for
the "default" provider, and in doing so, activate it if necessary,
thereby detecting that it's available...  and then immediately free
it, which could deactivate that provider, even though it should stay
available.

We solve this by incrementing the refcount for activated fallbacks one
extra time, thereby simulating an explicit OSSL_PROVIDER_load(), and
compensate for it with an extra ossl_provider_free() when emptying the
provider store.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11926)

1 files changed, 35 insertions, 7 deletions

diff --git a/crypto/provider_core.c b/crypto/provider_core.c
index f8aa5721b4..8b868fdb6b 100644
--- a/crypto/provider_core.c
+++ b/crypto/provider_core.c
@@ -42,7 +42,8 @@ struct provider_store_st;        /* Forward declaration */
 struct ossl_provider_st {
     /* Flag bits */
     unsigned int flag_initialized:1;
-    unsigned int flag_fallback:1;
+    unsigned int flag_fallback:1; /* Can be used as fallback */
+    unsigned int flag_activated_as_fallback:1;
 
     /* OpenSSL library side data */
     CRYPTO_REF_COUNT refcnt;
@@ -104,6 +105,24 @@ struct provider_store_st {
     unsigned int use_fallbacks:1;
 };
 
+/*
+ * provider_deactivate_free() is a wrapper around ossl_provider_free()
+ * that also makes sure that activated fallback providers are deactivated.
+ * This is simply done by freeing them an extra time, to compensate for the
+ * refcount that provider_activate_fallbacks() gives them.
+ * Since this is only called when the provider store is being emptied, we
+ * don't need to care about any lock.
+ */
+static void provider_deactivate_free(OSSL_PROVIDER *prov)
+{
+    int extra_free = (prov->flag_initialized
+                      && prov->flag_activated_as_fallback);
+
+    if (extra_free)
+        ossl_provider_free(prov);
+    ossl_provider_free(prov);
+}
+
 static void provider_store_free(void *vstore)
 {
     struct provider_store_st *store = vstore;
@@ -111,7 +130,7 @@ static void provider_store_free(void *vstore)
     if (store == NULL)
         return;
     OPENSSL_free(store->default_path);
-    sk_OSSL_PROVIDER_pop_free(store->providers, ossl_provider_free);
+    sk_OSSL_PROVIDER_pop_free(store->providers, provider_deactivate_free);
     CRYPTO_THREAD_lock_free(store->lock);
     OPENSSL_free(store);
 }
@@ -654,13 +673,22 @@ static void provider_activate_fallbacks(struct provider_store_st *store)
             OSSL_PROVIDER *prov = sk_OSSL_PROVIDER_value(store->providers, i);
 
             /*
-             * Note that we don't care if the activation succeeds or not.
-             * If it doesn't succeed, then any attempt to use any of the
-             * fallback providers will fail anyway.
+             * Activated fallback providers get an extra refcount, to
+             * simulate a regular load.
+             * Note that we don't care if the activation succeeds or not,
+             * other than to maintain a correct refcount.  If the activation
+             * doesn't succeed, then any future attempt to use the fallback
+             * provider will fail anyway.
              */
             if (prov->flag_fallback) {
-                activated_fallback_count++;
-                provider_activate(prov);
+                if (ossl_provider_up_ref(prov)) {
+                    if (!provider_activate(prov)) {
+                        ossl_provider_free(prov);
+                    } else {
+                        prov->flag_activated_as_fallback = 1;
+                        activated_fallback_count++;
+                    }
+                }
             }
         }