credential-factory: Enforce an upper limit when creating nested credentials

This mainly intended as defense-in-depth measure to avoid parsing
massively nested structures that could cause a call stack overflow due
to the massive recursion.  In particular PKCS#7 signed data is prone to
this as these can be nested basically infinitely.  When used in IKEv1 via
ENC_PKCS7_WRAPPED_X509 CERT payloads, our default of 10000 bytes for IKE
messages guards against this, but that's configurable and there might be
a chance for some bug that triggers problematic recursive parsing for
smaller input.

The upper limit is chosen arbitrarily, but there are currently no known
cases that require a depth of more than 10 levels.

src/libstrongswan/credentials/credential_factory.c

@@ -25,6 +25,11 @@
 #include <credentials/certificates/x509.h>
 #include <credentials/containers/container.h>
 
+/**
+ * Maximum depth/recursion before failing to create a credential.
+ */
+#define CREATE_MAX_DEPTH 10
+
 ENUM(credential_type_names, CRED_PRIVATE_KEY, CRED_CONTAINER,
 	"CRED_PRIVATE_KEY",
 	"CRED_PUBLIC_KEY",
@@ -137,6 +142,12 @@ METHOD(credential_factory_t, create, void*,
 	}
 
 	level = (uintptr_t)this->recursive->get(this->recursive);
+	if (level >= CREATE_MAX_DEPTH)
+	{
+		DBG1(DBG_LIB, "building %N - %N failed, reached depth limit (%d)",
+			 credential_type_names, type, names, subtype, CREATE_MAX_DEPTH);
+		return NULL;
+	}
 	this->recursive->set(this->recursive, (void*)level + 1);
 
 	this->lock->read_lock(this->lock);