From 53dfdcd8198d2b3201a23c4bad9190519ba918db Mon Sep 17 00:00:00 2001
From: Werner Lemberg <wl@gnu.org>
Date: Thu, 17 Mar 2022 19:24:16 +0100
Subject: [PATCH] [sfnt] Avoid invalid face index.

Fixes #1138.

* src/sfnt/sfobjs.c (sfnt_init_face), src/sfnt/sfwoff2.c (woff2_open_font):
Check `face_index` before decrementing.
---
 src/sfnt/sfobjs.c  | 2 +-
 src/sfnt/sfwoff2.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/sfnt/sfobjs.c b/src/sfnt/sfobjs.c
index f9d4d3858..9771c35df 100644
--- a/src/sfnt/sfobjs.c
+++ b/src/sfnt/sfobjs.c
@@ -566,7 +566,7 @@
     face_index = FT_ABS( face_instance_index ) & 0xFFFF;
 
     /* value -(N+1) requests information on index N */
-    if ( face_instance_index < 0 )
+    if ( face_instance_index < 0 && face_index > 0 )
       face_index--;
 
     if ( face_index >= face->ttc_header.count )
diff --git a/src/sfnt/sfwoff2.c b/src/sfnt/sfwoff2.c
index cb1e0664a..165b875e5 100644
--- a/src/sfnt/sfwoff2.c
+++ b/src/sfnt/sfwoff2.c
@@ -2085,7 +2085,7 @@
     /* Validate requested face index. */
     *num_faces = woff2.num_fonts;
     /* value -(N+1) requests information on index N */
-    if ( *face_instance_index < 0 )
+    if ( *face_instance_index < 0 && face_index > 0 )
       face_index--;
 
     if ( face_index >= woff2.num_fonts )
-- 
2.35.1