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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Zhang Boyang <zhangboyang.id@gmail.com>
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Date: Mon, 24 Oct 2022 07:15:41 +0800
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Subject: [PATCH] font: Harden grub_font_blit_glyph() and
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grub_font_blit_glyph_mirror()
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As a mitigation and hardening measure add sanity checks to
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grub_font_blit_glyph() and grub_font_blit_glyph_mirror(). This patch
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makes these two functions do nothing if target blitting area isn't fully
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contained in target bitmap. Therefore, if complex calculations in caller
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overflows and malicious coordinates are given, we are still safe because
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any coordinates which result in out-of-bound-write are rejected. However,
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this patch only checks for invalid coordinates, and doesn't provide any
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protection against invalid source glyph or destination glyph, e.g.
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mismatch between glyph size and buffer size.
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This hardening measure is designed to mitigate possible overflows in
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blit_comb(). If overflow occurs, it may return invalid bounding box
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during dry run and call grub_font_blit_glyph() with malicious
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coordinates during actual blitting. However, we are still safe because
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the scratch glyph itself is valid, although its size makes no sense, and
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any invalid coordinates are rejected.
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It would be better to call grub_fatal() if illegal parameter is detected.
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However, doing this may end up in a dangerous recursion because grub_fatal()
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would print messages to the screen and we are in the progress of drawing
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characters on the screen.
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Reported-by: Daniel Axtens <dja@axtens.net>
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Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
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Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
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(cherry picked from commit fcd7aa0c278f7cf3fb9f93f1a3966e1792339eb6)
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---
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grub-core/font/font.c | 14 ++++++++++++++
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1 file changed, 14 insertions(+)
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diff --git a/grub-core/font/font.c b/grub-core/font/font.c
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index 12a5f0d08c..29fbb94294 100644
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--- a/grub-core/font/font.c
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+++ b/grub-core/font/font.c
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@@ -1069,8 +1069,15 @@ static void
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grub_font_blit_glyph (struct grub_font_glyph *target,
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struct grub_font_glyph *src, unsigned dx, unsigned dy)
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{
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+ grub_uint16_t max_x, max_y;
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unsigned src_bit, tgt_bit, src_byte, tgt_byte;
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unsigned i, j;
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+
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+ /* Harden against out-of-bound writes. */
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+ if ((grub_add (dx, src->width, &max_x) || max_x > target->width) ||
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+ (grub_add (dy, src->height, &max_y) || max_y > target->height))
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+ return;
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+
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for (i = 0; i < src->height; i++)
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{
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src_bit = (src->width * i) % 8;
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@@ -1102,9 +1109,16 @@ grub_font_blit_glyph_mirror (struct grub_font_glyph *target,
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struct grub_font_glyph *src,
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unsigned dx, unsigned dy)
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{
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+ grub_uint16_t max_x, max_y;
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unsigned tgt_bit, src_byte, tgt_byte;
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signed src_bit;
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unsigned i, j;
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+
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+ /* Harden against out-of-bound writes. */
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+ if ((grub_add (dx, src->width, &max_x) || max_x > target->width) ||
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+ (grub_add (dy, src->height, &max_y) || max_y > target->height))
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+ return;
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+
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for (i = 0; i < src->height; i++)
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{
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src_bit = (src->width * i + src->width - 1) % 8;
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