[SECURITY PATCH 06/13] font: Fix integer overflow in BMP index

[Daniel Kiper]

From: Zhang Boyang <zhangboyang.id@gmail.com>

The BMP index (font->bmp_idx) is designed as a reverse lookup table of
char entries (font->char_index), in order to speed up lookups for BMP
chars (i.e. code < 0x10000). The values in BMP index are the subscripts
of the corresponding char entries, stored in grub_uint16_t, while 0xffff
means not found.

This patch fixes the problem of large subscript truncated to grub_uint16_t,
leading BMP index to return wrong char entry or report false miss. The
code now checks for bounds and uses BMP index as a hint, and fallbacks
to binary-search if necessary.

On the occasion add a comment about BMP index is initialized to 0xffff.

Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/font/font.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/grub-core/font/font.c b/grub-core/font/font.c
index 9e3e0a94e..e4cb0d867 100644
--- a/grub-core/font/font.c
+++ b/grub-core/font/font.c
@@ -300,6 +300,8 @@ load_font_index (grub_file_t file, grub_uint32_t 
sect_length, struct
   font->bmp_idx = grub_malloc (0x10000 * sizeof (grub_uint16_t));
   if (!font->bmp_idx)
     return 1;
+
+  /* Init the BMP index array to 0xffff. */
   grub_memset (font->bmp_idx, 0xff, 0x10000 * sizeof (grub_uint16_t));
 
 
@@ -328,7 +330,7 @@ load_font_index (grub_file_t file, grub_uint32_t 
sect_length, struct
          return 1;
        }
 
-      if (entry->code < 0x10000)
+      if (entry->code < 0x10000 && i < 0xffff)
        font->bmp_idx[entry->code] = i;
 
       last_code = entry->code;
@@ -696,9 +698,12 @@ find_glyph (const grub_font_t font, grub_uint32_t code)
   /* Use BMP index if possible.  */
   if (code < 0x10000 && font->bmp_idx)
     {
-      if (font->bmp_idx[code] == 0xffff)
-       return 0;
-      return &table[font->bmp_idx[code]];
+      if (font->bmp_idx[code] < 0xffff)
+       return &table[font->bmp_idx[code]];
+      /*
+       * When we are here then lookup in BMP index result in miss,
+       * fallthough to binary-search.
+       */
     }
 
   /* Do a binary search in `char_index', which is ordered by code point.  */
-- 
2.11.0