[SECURITY PATCH 070/117] term/gfxterm: Don't set up a font with glyphs that are too big

[Daniel Kiper]

From: Daniel Axtens <dja@axtens.net>

Catch the case where we have a font so big that it causes the number of
rows or columns to be 0. Currently we continue and allocate a
virtual_screen.text_buffer of size 0. We then try to use that for glpyhs
and things go badly.

On the emu platform, malloc() may give us a valid pointer, in which case
we'll access heap memory which we shouldn't. Alternatively, it may give us
NULL, in which case we'll crash. For other platforms, if I understand
grub_memalign() correctly, we will receive a valid but small allocation
that we will very likely later overrun.

Prevent the creation of a virtual screen that isn't at least 40 cols
by 12 rows. This is arbitrary, but it seems that if your width or height
is half a standard 80x24 terminal, you're probably going to struggle to
read anything anyway.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/term/gfxterm.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/grub-core/term/gfxterm.c b/grub-core/term/gfxterm.c
index af7c090a3..b40fcce91 100644
--- a/grub-core/term/gfxterm.c
+++ b/grub-core/term/gfxterm.c
@@ -232,6 +232,15 @@ grub_virtual_screen_setup (unsigned int x, unsigned int y,
   virtual_screen.columns = virtual_screen.width / 
virtual_screen.normal_char_width;
   virtual_screen.rows = virtual_screen.height / 
virtual_screen.normal_char_height;
 
+  /*
+   * There must be a minimum number of rows and columns for the screen to
+   * make sense. Arbitrarily pick half of 80x24. If either dimensions is 0
+   * we would allocate 0 bytes for the text_buffer.
+   */
+  if (virtual_screen.columns < 40 || virtual_screen.rows < 12)
+    return grub_error (GRUB_ERR_BAD_FONT,
+                      "font: glyphs too large to fit on screen");
+
   /* Allocate memory for text buffer.  */
   virtual_screen.text_buffer =
     (struct grub_colored_char *) grub_malloc (virtual_screen.columns
-- 
2.11.0