[SECURITY PATCH 084/117] io/gzio: Catch missing values in huft_build() and bail

[Daniel Kiper]

From: Daniel Axtens <dja@axtens.net>

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 4236f0fd4..19adebeed 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);
-- 
2.11.0