[SECURITY PATCH 02/13] font: Fix size overflow in grub_font_get_glyph_internal()

[Daniel Kiper]

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

The length of memory allocation and file read may overflow. This patch
fixes the problem by using safemath macros.

There is a lot of code repetition like "(x * y + 7) / 8". It is unsafe
if overflow happens. This patch introduces grub_video_bitmap_calc_1bpp_bufsz().
It is safe replacement for such code. It has safemath-like prototype.

This patch also introduces grub_cast(value, pointer), it casts value to
typeof(*pointer) then store the value to *pointer. It returns true when
overflow occurs or false if there is no overflow. The semantics of arguments
and return value are designed to be consistent with other safemath macros.

Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/font/font.c   | 17 +++++++++++++----
 include/grub/bitmap.h   | 18 ++++++++++++++++++
 include/grub/safemath.h |  2 ++
 3 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/grub-core/font/font.c b/grub-core/font/font.c
index 756ca0abf..e781521a7 100644
--- a/grub-core/font/font.c
+++ b/grub-core/font/font.c
@@ -739,7 +739,8 @@ grub_font_get_glyph_internal (grub_font_t font, 
grub_uint32_t code)
       grub_int16_t xoff;
       grub_int16_t yoff;
       grub_int16_t dwidth;
-      int len;
+      grub_ssize_t len;
+      grub_size_t sz;
 
       if (index_entry->glyph)
        /* Return cached glyph.  */
@@ -768,9 +769,17 @@ grub_font_get_glyph_internal (grub_font_t font, 
grub_uint32_t code)
          return 0;
        }
 
-      len = (width * height + 7) / 8;
-      glyph = grub_malloc (sizeof (struct grub_font_glyph) + len);
-      if (!glyph)
+      /* Calculate real struct size of current glyph. */
+      if (grub_video_bitmap_calc_1bpp_bufsz (width, height, &len) ||
+         grub_add (sizeof (struct grub_font_glyph), len, &sz))
+       {
+         remove_font (font);
+         return 0;
+       }
+
+      /* Allocate and initialize the glyph struct. */
+      glyph = grub_malloc (sz);
+      if (glyph == NULL)
        {
          remove_font (font);
          return 0;
diff --git a/include/grub/bitmap.h b/include/grub/bitmap.h
index 149d37bfe..431048936 100644
--- a/include/grub/bitmap.h
+++ b/include/grub/bitmap.h
@@ -23,6 +23,7 @@
 #include <grub/symbol.h>
 #include <grub/types.h>
 #include <grub/video.h>
+#include <grub/safemath.h>
 
 #define IMAGE_HW_MAX_PX                16384
 
@@ -81,6 +82,23 @@ grub_video_bitmap_get_height (struct grub_video_bitmap 
*bitmap)
   return bitmap->mode_info.height;
 }
 
+/*
+ * Calculate and store the size of data buffer of 1bit bitmap in result.
+ * Equivalent to "*result = (width * height + 7) / 8" if no overflow occurs.
+ * Return true when overflow occurs or false if there is no overflow.
+ * This function is intentionally implemented as a macro instead of
+ * an inline function. Although a bit awkward, it preserves data types for
+ * safemath macros and reduces macro side effects as much as possible.
+ *
+ * XXX: Will report false overflow if width * height > UINT64_MAX.
+ */
+#define grub_video_bitmap_calc_1bpp_bufsz(width, height, result) \
+({ \
+  grub_uint64_t _bitmap_pixels; \
+  grub_mul ((width), (height), &_bitmap_pixels) ? 1 : \
+    grub_cast (_bitmap_pixels / GRUB_CHAR_BIT + !!(_bitmap_pixels % 
GRUB_CHAR_BIT), (result)); \
+})
+
 void EXPORT_FUNC (grub_video_bitmap_get_mode_info) (struct grub_video_bitmap 
*bitmap,
                                                    struct grub_video_mode_info 
*mode_info);
 
diff --git a/include/grub/safemath.h b/include/grub/safemath.h
index 51290d355..fbd9b5925 100644
--- a/include/grub/safemath.h
+++ b/include/grub/safemath.h
@@ -30,6 +30,8 @@
 #define grub_sub(a, b, res)    __builtin_sub_overflow(a, b, res)
 #define grub_mul(a, b, res)    __builtin_mul_overflow(a, b, res)
 
+#define grub_cast(a, res)      grub_add ((a), 0, (res))
+
 #else
 #error gcc 5.1 or newer or clang 8.0 or newer is required
 #endif
-- 
2.11.0