[freetype2] master 3829fdaae: Avoid overflow in COLR bounds checks.

[Werner Lemberg]

branch: master
commit 3829fdaae5f12590f93807e9bcb866be131a201a
Author: Ben Wagner <bungeman@chromium.org>
Commit: Ben Wagner <bungeman@chromium.org>

    Avoid overflow in COLR bounds checks.
    
    The values read into `base_glyphs_offset_v1` and `layer_offset_v1` may
    be in the range 0xFFFFFFFD-0xFFFFFFFF. On systems where `unsigned long`
    is 32 bits adding 4 to such values will wrap and pass bounds checks but
    accessing values at such offsets will be out of bounds.
    
    On the other hand `table_size` has already been tested to be at least
    `COLRV1_HEADER_SIZE` (34) so it is safe to subtract 4 from it.
    
    * src/sfnt/ttcolr.c (tt_face_load_colr): subtract 4 from `table_size`
    instead of adding 4 to font data offsets in bounds checks
    
    Fixes: https://crbug.com/1469348
---
 src/sfnt/ttcolr.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/sfnt/ttcolr.c b/src/sfnt/ttcolr.c
index 69ccf0ee7..281e7135e 100644
--- a/src/sfnt/ttcolr.c
+++ b/src/sfnt/ttcolr.c
@@ -229,7 +229,7 @@
 
       base_glyphs_offset_v1 = FT_NEXT_ULONG( p );
 
-      if ( base_glyphs_offset_v1 + 4 >= table_size )
+      if ( base_glyphs_offset_v1 >= table_size - 4 )
         goto InvalidTable;
 
       p1                 = (FT_Byte*)( table + base_glyphs_offset_v1 );
@@ -249,7 +249,7 @@
 
       if ( layer_offset_v1 )
       {
-        if ( layer_offset_v1 + 4 >= table_size )
+        if ( layer_offset_v1 >= table_size - 4 )
           goto InvalidTable;
 
         p1            = (FT_Byte*)( table + layer_offset_v1 );