[core] harden Glyph PBF parsing

3 files changed, 41 insertions, 12 deletions

diff --git a/src/mbgl/text/glyph.hpp b/src/mbgl/text/glyph.hpp
index d07fbdff21..2bf1448492 100644
--- a/src/mbgl/text/glyph.hpp
+++ b/src/mbgl/text/glyph.hpp
@@ -77,9 +77,13 @@ class Shaping {
 
 class SDFGlyph {
 public:
+    // We're using this value throughout the Mapbox GL ecosystem. If this is different, the glyphs
+    // also need to be reencoded.
+    static constexpr const uint8_t borderSize = 3;
+
     uint32_t id = 0;
 
-    // A signed distance field of the glyph with a border of 3 pixels.
+    // A signed distance field of the glyph with a border (see above).
     std::string bitmap;
 
     // Glyph metrics
diff --git a/src/mbgl/text/glyph_atlas.cpp b/src/mbgl/text/glyph_atlas.cpp
index 5d30dacdce..17b3e7e482 100644
--- a/src/mbgl/text/glyph_atlas.cpp
+++ b/src/mbgl/text/glyph_atlas.cpp
@@ -107,9 +107,6 @@ Rect<uint16_t> GlyphAtlas::addGlyph(uintptr_t tileUID,
                                     const FontStack& fontStack,
                                     const SDFGlyph& glyph)
 {
-    // Use constant value for now.
-    const uint8_t buffer = 3;
-
     std::map<uint32_t, GlyphValue>& face = index[fontStack];
     auto it = face.find(glyph.id);
 
@@ -125,8 +122,8 @@ Rect<uint16_t> GlyphAtlas::addGlyph(uintptr_t tileUID,
         return Rect<uint16_t>{ 0, 0, 0, 0 };
     }
 
-    uint16_t buffered_width = glyph.metrics.width + buffer * 2;
-    uint16_t buffered_height = glyph.metrics.height + buffer * 2;
+    uint16_t buffered_width = glyph.metrics.width + SDFGlyph::borderSize * 2;
+    uint16_t buffered_height = glyph.metrics.height + SDFGlyph::borderSize * 2;
 
     // Add a 1px border around every image.
     const uint16_t padding = 1;
diff --git a/src/mbgl/text/glyph_pbf.cpp b/src/mbgl/text/glyph_pbf.cpp
index 21edfe436c..cdeac57984 100644
--- a/src/mbgl/text/glyph_pbf.cpp
+++ b/src/mbgl/text/glyph_pbf.cpp
@@ -12,9 +12,11 @@
 
 #include <protozero/pbf_reader.hpp>
 
+namespace mbgl {
+
 namespace {
 
-void parseGlyphPBF(mbgl::GlyphSet& glyphSet, const std::string& data) {
+void parseGlyphPBF(GlyphSet& glyphSet, const GlyphRange& glyphRange, const std::string& data) {
     protozero::pbf_reader glyphs_pbf(data);
 
     while (glyphs_pbf.next(1)) {
@@ -22,30 +24,39 @@ void parseGlyphPBF(mbgl::GlyphSet& glyphSet, const std::string& data) {
         while (fontstack_pbf.next(3)) {
             auto glyph_pbf = fontstack_pbf.get_message();
 
-            mbgl::SDFGlyph glyph;
+            SDFGlyph glyph;
+
+            bool hasID = false, hasWidth = false, hasHeight = false, hasLeft = false,
+                 hasTop = false, hasAdvance = false;
 
             while (glyph_pbf.next()) {
                 switch (glyph_pbf.tag()) {
                 case 1: // id
                     glyph.id = glyph_pbf.get_uint32();
+                    hasID = true;
                     break;
                 case 2: // bitmap
                     glyph.bitmap = glyph_pbf.get_string();
                     break;
                 case 3: // width
                     glyph.metrics.width = glyph_pbf.get_uint32();
+                    hasWidth = true;
                     break;
                 case 4: // height
                     glyph.metrics.height = glyph_pbf.get_uint32();
+                    hasHeight = true;
                     break;
                 case 5: // left
                     glyph.metrics.left = glyph_pbf.get_sint32();
+                    hasLeft = true;
                     break;
                 case 6: // top
                     glyph.metrics.top = glyph_pbf.get_sint32();
+                    hasTop = true;
                     break;
                 case 7: // advance
                     glyph.metrics.advance = glyph_pbf.get_uint32();
+                    hasAdvance = true;
                     break;
                 default:
                     glyph_pbf.skip();
@@ -53,15 +64,32 @@ void parseGlyphPBF(mbgl::GlyphSet& glyphSet, const std::string& data) {
                 }
             }
 
-            glyphSet.insert(glyph.id, std::move(glyph));
+            // If the area of width/height is non-zero, we need to adjust the expected size
+            // with the implicit border size, otherwise we expect there to be no bitmap at all.
+            const uint32_t expectedBitmapSize =
+                glyph.metrics.width && glyph.metrics.height
+                    ? (glyph.metrics.width + 2 * SDFGlyph::borderSize) *
+                          (glyph.metrics.height + 2 * SDFGlyph::borderSize)
+                    : 0;
+
+            // Only treat this glyph as a correct glyph if it has all required fields, and if
+            // the bitmap has the correct length. It also needs to satisfy a few metrics conditions
+            // that ensure that the glyph isn't bogus. All other glyphs are malformed.
+            // We're also discarding all glyphs that are outside the expected glyph range.
+            if (hasID && hasWidth && hasHeight && hasLeft && hasTop && hasAdvance &&
+                glyph.metrics.width < 256 && glyph.metrics.height < 256 &&
+                glyph.metrics.left >= -128 && glyph.metrics.left < 128 &&
+                glyph.metrics.top >= -128 && glyph.metrics.top < 128 &&
+                glyph.metrics.advance < 256 && glyph.bitmap.size() == expectedBitmapSize &&
+                glyph.id >= glyphRange.first && glyph.id <= glyphRange.second) {
+                glyphSet.insert(glyph.id, std::move(glyph));
+            }
         }
     }
 }
 
 } // namespace
 
-namespace mbgl {
-
 GlyphPBF::GlyphPBF(GlyphAtlas* atlas,
                    const FontStack& fontStack,
                    const GlyphRange& glyphRange,
@@ -79,7 +107,7 @@ GlyphPBF::GlyphPBF(GlyphAtlas* atlas,
             observer->onGlyphsLoaded(fontStack, glyphRange);
         } else {
             try {
-                parseGlyphPBF(**atlas->getGlyphSet(fontStack), *res.data);
+                parseGlyphPBF(**atlas->getGlyphSet(fontStack), glyphRange, *res.data);
             } catch (...) {
                 observer->onGlyphsError(fontStack, glyphRange, std::current_exception());
                 return;