From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Daniel Axtens <dja@axtens.net>
Date: Fri, 15 Jan 2021 13:29:53 +1100
Subject: [PATCH] video/readers/jpeg: Catch OOB reads/writes in
 grub_jpeg_decode_du()

The key line is:

  du[jpeg_zigzag_order[pos]] = val * (int) data->quan_table[qt][pos];

jpeg_zigzag_order is grub_uint8_t[64].

I don't understand JPEG decoders quite well enough to explain what's
going on here. However, I observe sometimes pos=64, which leads to an
OOB read of the jpeg_zigzag_order global then an OOB write to du.
That leads to various unpleasant memory corruption conditions.

Catch where pos >= ARRAY_SIZE(jpeg_zigzag_order) and bail.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/video/readers/jpeg.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/grub-core/video/readers/jpeg.c b/grub-core/video/readers/jpeg.c
index 856573996ac..28c1d62515e 100644
--- a/grub-core/video/readers/jpeg.c
+++ b/grub-core/video/readers/jpeg.c
@@ -526,6 +526,14 @@ grub_jpeg_decode_du (struct grub_jpeg_data *data, int id, jpeg_data_unit_t du)
       val = grub_jpeg_get_number (data, num & 0xF);
       num >>= 4;
       pos += num;
+
+      if (pos >= ARRAY_SIZE (jpeg_zigzag_order))
+	{
+	  grub_error (GRUB_ERR_BAD_FILE_TYPE,
+		      "jpeg: invalid position in zigzag order!?");
+	  return;
+	}
+
       du[jpeg_zigzag_order[pos]] = val * (int) data->quan_table[qt][pos];
       pos++;
     }