From 210245129c932dc9e1c2748d9d35524fb95b5042 Mon Sep 17 00:00:00 2001
From: Daniel Axtens <dja@axtens.net>
Date: Tue, 6 Jul 2021 23:25:07 +1000
Subject: 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

Reference:https://git.savannah.gnu.org/cgit/grub.git/commit/?id=210245129c932dc9e1c2748d9d35524fb95b5042
Conflict:NA

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 a3161e2..d7ed5aa 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];
 
-- 
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