File 0010-video-readers-png-Avoid-heap-OOB-R-W-inserting-huff-.patch of Package grub2.28282

From 8ebb6943eae81d3c31963bbae42d5d1f168c8dd5 Mon Sep 17 00:00:00 2001
From: Daniel Axtens <dja@axtens.net>
Date: Tue, 6 Jul 2021 23:25:07 +1000
Subject: [PATCH 10/32] video/readers/png: Avoid heap OOB R/W inserting huff
 table items

In fuzzing we observed crashes where a code would attempt to be inserted
into a huffman table before the start, leading to a set of heap OOB reads
and writes as table entries with negative indices were shifted around and
the new code written in.

Catch the case where we would underflow the array and bail.

Fixes: CVE-2021-3696

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/video/readers/png.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/grub-core/video/readers/png.c b/grub-core/video/readers/png.c
index a3161e25b6..d7ed5aa6cf 100644
--- a/grub-core/video/readers/png.c
+++ b/grub-core/video/readers/png.c
@@ -438,6 +438,13 @@ grub_png_insert_huff_item (struct huff_table *ht, int code, int len)
   for (i = len; i < ht->max_length; i++)
     n += ht->maxval[i];
 
+  if (n > ht->num_values)
+    {
+      grub_error (GRUB_ERR_BAD_FILE_TYPE,
+		  "png: out of range inserting huffman table item");
+      return;
+    }
+
   for (i = 0; i < n; i++)
     ht->values[ht->num_values - i] = ht->values[ht->num_values - i - 1];
 
-- 
2.34.1