From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Daniel Axtens <dja@axtens.net>
Date: Thu, 21 Jan 2021 12:20:49 +1100
Subject: [PATCH] io/gzio: Catch missing values in huft_build() and bail

In huft_build(), "v" is a table of values in order of bit length.
The code later (when setting up table entries in "r") assumes that all
elements of this array corresponding to a code are initialized and less
than N_MAX. However, it doesn't enforce this.

With sufficiently manipulated inputs (e.g. from fuzzing), there can be
elements of "v" that are not filled. Therefore a lookup into "e" or "d"
will use an uninitialized value. This can lead to an invalid/OOB read on
those values, often leading to a crash.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/io/gzio.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/grub-core/io/gzio.c b/grub-core/io/gzio.c
index 389fd345488..fa7d9990b84 100644
--- a/grub-core/io/gzio.c
+++ b/grub-core/io/gzio.c
@@ -507,6 +507,7 @@ huft_build (unsigned *b,	/* code lengths in bits (all assumed <= BMAX) */
     }
 
   /* Make a table of values in order of bit lengths */
+  grub_memset (v, N_MAX, ARRAY_SIZE (v));
   p = b;
   i = 0;
   do
@@ -588,11 +589,18 @@ huft_build (unsigned *b,	/* code lengths in bits (all assumed <= BMAX) */
 	      r.v.n = (ush) (*p);	/* simple code is just the value */
 	      p++;		/* one compiler does not like *p++ */
 	    }
-	  else
+	  else if (*p < N_MAX)
 	    {
 	      r.e = (uch) e[*p - s];	/* non-simple--look up in lists */
 	      r.v.n = d[*p++ - s];
 	    }
+	  else
+	    {
+	      /* Detected an uninitialised value, abort. */
+	      if (h)
+		huft_free (u[0]);
+	      return 2;
+	    }
 
 	  /* fill code-like entries with r */
 	  f = 1 << (k - w);