font: Fix integer overflow in BMP index

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>

debian/patches/cve_2022_2601/0007-font-Fix-integer-overflow-in-BMP-index.patch

@@ -0,0 +1,65 @@
+From afda8b60ba0712abe01ae1e64c5f7a067a0e6492 Mon Sep 17 00:00:00 2001
+From: Zhang Boyang <zhangboyang.id@gmail.com>
+Date: Mon, 15 Aug 2022 02:04:58 +0800
+Subject: [PATCH 07/14] font: Fix integer overflow in BMP index
+
+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.  */
+-- 
+2.30.2
+

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)
+      if (font->bmp_idx[code] < 0xffff)
-	return 0;
+	return &table[font->bmp_idx[code]];
-      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.  */