Bug 1767974 - Add preferences for ECH GREASE Mode. r=keeler,dragana

Differential Revision: https://phabricator.services.mozilla.com/D145608

modules/libpref/init/StaticPrefList.yaml

@@ -12193,6 +12193,7 @@
 #endif
   mirror: always
 
+
 # Whether or not OCSP is required.
 # true => hard-fail (if an OCSP request times out, stop the connection)
 # false => soft-fail (if an OCSP request times out, continue the connection)
@@ -12340,6 +12341,19 @@
   value: false
   mirror: always
 
+# Probability of GREASEing a TLS connection with ECH (0-100)
+# 0 means never GREASE, 100 means always GREASE
+- name: security.tls.ech.grease_probability
+  type: RelaxedAtomicUint32
+  value: 0
+  mirror: always
+
+# ECH GREASE Padding target (1-255)
+- name: security.tls.ech.grease_size
+  type: RelaxedAtomicUint32
+  value: 100
+  mirror: always
+
 - name: security.tls.hello_downgrade_check
   type: RelaxedAtomicBool
   value: true

security/manager/ssl/nsNSSIOLayer.cpp

@@ -23,6 +23,8 @@
 #include "mozilla/DebugOnly.h"
 #include "mozilla/Logging.h"
 #include "mozilla/Preferences.h"
+#include "mozilla/RandomNum.h"
+#include "mozilla/StaticPrefs_security.h"
 #include "mozilla/Telemetry.h"
 #include "mozilla/ipc/BackgroundChild.h"
 #include "mozilla/ipc/PBackgroundChild.h"
@@ -41,6 +43,7 @@
 #include "nsContentUtils.h"
 #include "nsIClientAuthDialogs.h"
 #include "nsISocketProvider.h"
+#include "nsISocketTransport.h"
 #include "nsIWebProgressListener.h"
 #include "nsNSSCertHelper.h"
 #include "nsNSSComponent.h"
@@ -2687,6 +2690,28 @@ static nsresult nsSSLIOLayerSetOptions(PRFileDesc* fd, bool forSTARTTLS,
     }
   }
 
+  // Enable ECH GREASE if suitable. Has no impact if 'real' ECH is being used.
+  if (range.max >= SSL_LIBRARY_VERSION_TLS_1_3 &&
+      !(infoObject->GetProviderFlags() & (nsISocketProvider::BE_CONSERVATIVE |
+                                          nsISocketTransport::DONT_TRY_ECH)) &&
+      StaticPrefs::security_tls_ech_grease_probability()) {
+    if ((RandomUint64().valueOr(0) % 100) >=
+        100 - StaticPrefs::security_tls_ech_grease_probability()) {
+      MOZ_LOG(gPIPNSSLog, LogLevel::Debug,
+              ("[%p] nsSSLIOLayerSetOptions: enabling TLS ECH Grease\n", fd));
+      if (SECSuccess != SSL_EnableTls13GreaseEch(fd, PR_TRUE)) {
+        return NS_ERROR_FAILURE;
+      }
+      // ECH Padding can be between 1 and 255
+      if (SECSuccess !=
+          SSL_SetTls13GreaseEchSize(
+              fd, std::clamp(StaticPrefs::security_tls_ech_grease_size(), 1U,
+                             255U))) {
+        return NS_ERROR_FAILURE;
+      }
+    }
+  }
+
   // Include a modest set of named groups.
   // Please change getKeaGroupName in nsNSSCallbacks.cpp when changing the list
   // here.