From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001

From: Daniel Axtens <dja@axtens.net>

Date: Tue, 6 Jul 2021 23:25:07 +1000

Subject: [PATCH] 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>

(cherry picked from commit 1ae9a91d42cb40da8a6f11fac65541858e340afa)

---

 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];