[PATCH] sfnt2fon: Prevent reading past allocated buffer

[janisozaur at gmail.com]

From: Michał Janiszewski <janisozaur at gmail.com>

Some fonts (e.g. Courier) may cause sfnt2fon try reading past end of
heap-allocated buffer. Rather than reading from uninitialised memory,
return 0 in such case.

Visual tests on .fon render of this font (using FontForge) shown no
artifacts.
---
 tools/sfnt2fon/sfnt2fon.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/sfnt2fon/sfnt2fon.c b/tools/sfnt2fon/sfnt2fon.c
index 25f0763872..fb125bbeaf 100644
--- a/tools/sfnt2fon/sfnt2fon.c
+++ b/tools/sfnt2fon/sfnt2fon.c
@@ -570,11 +570,14 @@ static struct fontinfo *fill_fontinfo( const char *face_name, int ppem, int enc,
                 else
                     left_byte = face->glyph->bitmap.buffer[(y - (ascent - face->glyph->bitmap_top)) * face->glyph->bitmap.pitch + x - x_off - 1];
 
+                int x_idx = (y - (ascent - face->glyph->bitmap_top)) * face->glyph->bitmap.pitch + x - x_off;
+                // FreeType allocates that many bytes for bitmap, but some fonts (e.g. Courier) might try reading past the buffer
+                int x_limit = face->glyph->bitmap.rows * face->glyph->bitmap.pitch;
                 /* On the last non-trivial output byte (x == x_end) have we got one or two input bytes */
-                if(x == x_end && (face->glyph->bitmap_left % 8 != 0) && ((face->glyph->bitmap.width % 8 == 0) || (x != (((face->glyph->bitmap.width) & ~0x7) + face->glyph->bitmap_left) / 8)))
+                if((x_idx >= x_limit) || (x == x_end && (face->glyph->bitmap_left % 8 != 0) && ((face->glyph->bitmap.width % 8 == 0) || (x != (((face->glyph->bitmap.width) & ~0x7) + face->glyph->bitmap_left) / 8))))
                     right_byte = 0;
                 else
-                    right_byte = face->glyph->bitmap.buffer[(y - (ascent - face->glyph->bitmap_top)) * face->glyph->bitmap.pitch + x - x_off];
+                    right_byte = face->glyph->bitmap.buffer[x_idx];
 
                 byte = (left_byte << (8 - (face->glyph->bitmap_left & 7))) & 0xff;
                 byte |= ((right_byte >> (face->glyph->bitmap_left & 7)) & 0xff);
-- 
2.18.0