font: Fix an integer underflow in blit_comb()

The expression (ctx.bounds.height - combining_glyphs[i]->height) / 2 may
evaluate to a very big invalid value even if both ctx.bounds.height and
combining_glyphs[i]->height are small integers. For example, if
ctx.bounds.height is 10 and combining_glyphs[i]->height is 12, this
expression evaluates to 2147483647 (expected -1). This is because
coordinates are allowed to be negative but ctx.bounds.height is an
unsigned int. So, the subtraction operates on unsigned ints and
underflows to a very big value. The division makes things even worse.
The quotient is still an invalid value even if converted back to int.

This patch fixes the problem by casting ctx.bounds.height to int. As
a result the subtraction will operate on int and grub_uint16_t which
will be promoted to an int. So, the underflow will no longer happen. Other
uses of ctx.bounds.height (and ctx.bounds.width) are also casted to int,
to ensure coordinates are always calculated on signed integers.

Fixes: CVE-2022-3775
Reported-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

diff --git a/grub-core/font/font.c b/grub-core/font/font.c
index abd412a5e5bd8ec62a0526184b803075ff481ff2..3d3d803e81b084bbec625be05ffc272058798074 100644 (file)
--- a/grub-core/font/font.c
+++ b/grub-core/font/font.c
@@ -1203,12 +1203,12 @@ blit_comb (const struct grub_unicode_glyph *glyph_id,
   ctx.bounds.height = main_glyph->height;
 
   above_rightx = main_glyph->offset_x + main_glyph->width;
-  above_righty = ctx.bounds.y + ctx.bounds.height;
+  above_righty = ctx.bounds.y + (int) ctx.bounds.height;
 
   above_leftx = main_glyph->offset_x;
-  above_lefty = ctx.bounds.y + ctx.bounds.height;
+  above_lefty = ctx.bounds.y + (int) ctx.bounds.height;
 
-  below_rightx = ctx.bounds.x + ctx.bounds.width;
+  below_rightx = ctx.bounds.x + (int) ctx.bounds.width;
   below_righty = ctx.bounds.y;
 
   comb = grub_unicode_get_comb (glyph_id);
@@ -1221,7 +1221,7 @@ blit_comb (const struct grub_unicode_glyph *glyph_id,
 
       if (!combining_glyphs[i])
        continue;
-      targetx = (ctx.bounds.width - combining_glyphs[i]->width) / 2 + ctx.bounds.x;
+      targetx = ((int) ctx.bounds.width - combining_glyphs[i]->width) / 2 + ctx.bounds.x;
       /* CGJ is to avoid diacritics reordering. */
       if (comb[i].code
          == GRUB_UNICODE_COMBINING_GRAPHEME_JOINER)
@@ -1231,8 +1231,8 @@ blit_comb (const struct grub_unicode_glyph *glyph_id,
        case GRUB_UNICODE_COMB_OVERLAY:
          do_blit (combining_glyphs[i],
                   targetx,
-                  (ctx.bounds.height - combining_glyphs[i]->height) / 2
-                  - (ctx.bounds.height + ctx.bounds.y), &ctx);
+                  ((int) ctx.bounds.height - combining_glyphs[i]->height) / 2
+                  - ((int) ctx.bounds.height + ctx.bounds.y), &ctx);
          if (min_devwidth < combining_glyphs[i]->width)
            min_devwidth = combining_glyphs[i]->width;
          break;
@@ -1305,7 +1305,7 @@ blit_comb (const struct grub_unicode_glyph *glyph_id,
          /* Fallthrough.  */
        case GRUB_UNICODE_STACK_ATTACHED_ABOVE:
          do_blit (combining_glyphs[i], targetx,
-                  -(ctx.bounds.height + ctx.bounds.y + space
+                  -((int) ctx.bounds.height + ctx.bounds.y + space
                     + combining_glyphs[i]->height), &ctx);
          if (min_devwidth < combining_glyphs[i]->width)
            min_devwidth = combining_glyphs[i]->width;
@@ -1313,7 +1313,7 @@ blit_comb (const struct grub_unicode_glyph *glyph_id,
 
        case GRUB_UNICODE_COMB_HEBREW_DAGESH:
          do_blit (combining_glyphs[i], targetx,
-                  -(ctx.bounds.height / 2 + ctx.bounds.y
+                  -((int) ctx.bounds.height / 2 + ctx.bounds.y
                     + combining_glyphs[i]->height / 2), &ctx);
          if (min_devwidth < combining_glyphs[i]->width)
            min_devwidth = combining_glyphs[i]->width;