Bug 1863574 - patch 2 - Create an RFPTarget option to control whether the resolution of CSS generics is subject to font-visibility restrictions. r=jwatt

Differential Revision: https://phabricator.services.mozilla.com/D195426

gfx/thebes/gfxPlatformFontList.cpp

@@ -38,6 +38,7 @@
 #include "mozilla/dom/ContentChild.h"
 #include "mozilla/dom/ContentParent.h"
 #include "mozilla/dom/ContentProcessMessageManager.h"
+#include "mozilla/dom/Document.h"
 #include "mozilla/gfx/2D.h"
 #include "mozilla/ipc/FileDescriptorUtils.h"
 #include "mozilla/ResultExtensions.h"
@@ -1535,6 +1536,17 @@ bool gfxPlatformFontList::FindAndAddFamiliesLocked(
   FontVisibility visibilityLevel =
       aPresContext ? aPresContext->GetFontVisibility() : FontVisibility::User;
 
+  // If this font lookup is the result of resolving a CSS generic (not a direct
+  // font-family request by the page), and RFP settings allow generics to be
+  // unrestricted, bump the effective visibility level applied here so as to
+  // allow user-installed fonts to be used.
+  if (visibilityLevel < FontVisibility::User &&
+      aGeneric != StyleGenericFontFamily::None &&
+      !aPresContext->Document()->ShouldResistFingerprinting(
+          RFPTarget::FontVisibilityRestrictGenerics)) {
+      visibilityLevel = FontVisibility::User;
+  }
+
   if (SharedFontList()) {
     fontlist::Family* family = SharedFontList()->FindFamily(key);
     // If not found, and other family names have not yet been initialized,

layout/base/nsPresContext.h

@@ -161,7 +161,7 @@ class nsPresContext : public nsISupports, public mozilla::SupportsWeakPtr {
    */
   nsresult Init(nsDeviceContext* aDeviceContext);
 
-  /*
+  /**
    * Initialize the font cache if it hasn't been initialized yet.
    * (Needed for stylo)
    */
@@ -169,7 +169,18 @@ class nsPresContext : public nsISupports, public mozilla::SupportsWeakPtr {
 
   void UpdateFontCacheUserFonts(gfxUserFontSet* aUserFontSet);
 
+  /**
+   * Return the font visibility level to be applied to this context,
+   * potentially blocking user-installed or non-standard fonts from being
+   * used by web content.
+   * Note that depending on ResistFingerprinting options, the caller may
+   * override this value when resolving CSS <generic-family> keywords.
+   */
   FontVisibility GetFontVisibility() const { return mFontVisibility; }
+
+  /**
+   * Log a message to the console about a font request being blocked.
+   */
   void ReportBlockedFontFamily(const mozilla::fontlist::Family& aFamily);
   void ReportBlockedFontFamily(const gfxFontFamily& aFamily);
 

toolkit/components/resistfingerprinting/RFPTargets.inc

@@ -91,6 +91,9 @@ ITEM_VALUE(CSSPointerCapabilities,      1llu << 57)
 ITEM_VALUE(WebGLRenderCapability,       1llu << 58)
 ITEM_VALUE(WebGLRenderInfo,             1llu << 59)
 ITEM_VALUE(SiteSpecificZoom,            1llu << 60)
+// Are font visibility restrictions applied when resolving a CSS <generic-family>?
+// (This may block the fonts selected in Preferences from actually being used.)
+ITEM_VALUE(FontVisibilityRestrictGenerics, 1llu << 61)
 
 // !!! Don't forget to update kDefaultFingerprintingProtections in nsRFPService.cpp
 // if necessary.