[gdk-pixbuf: 1/2] gif: Fix LZW decoder accepting invalid LZW code.

[Emmanuele Bassi <ebassi src gnome org>]

commit bdd3acbd48a575d418ba6bf1b32d7bda2fae1c81
Author: Robert Ancell <robert ancell canonical com>
Date:   Mon Nov 30 12:26:12 2020 +1300

    gif: Fix LZW decoder accepting invalid LZW code.
    
    The code value after a reset wasn't being validated, which means we would
    accept invalid codes. This could cause an infinite loop in the decoder.
    
    Fixes CVE-2020-29385
    
    Fixes https://gitlab.gnome.org/GNOME/gdk-pixbuf/-/issues/164

 gdk-pixbuf/lzw.c                    |  13 +++++++------
 tests/test-images/fail/hang_114.gif | Bin 0 -> 5561 bytes
 2 files changed, 7 insertions(+), 6 deletions(-)
---
diff --git a/gdk-pixbuf/lzw.c b/gdk-pixbuf/lzw.c
index 9e052a6f7..105daf2b1 100644
--- a/gdk-pixbuf/lzw.c
+++ b/gdk-pixbuf/lzw.c
@@ -195,19 +195,20 @@ lzw_decoder_feed (LZWDecoder *self,
                                 if (self->last_code != self->clear_code && self->code_table_size < 
MAX_CODES) {
                                         if (self->code < self->code_table_size)
                                                 add_code (self, self->code);
-                                        else if (self->code == self->code_table_size)
+                                        else
                                                 add_code (self, self->last_code);
-                                        else {
-                                                /* Invalid code received - just stop here */
-                                                self->last_code = self->eoi_code;
-                                                return output_length;
-                                        }
 
                                         /* When table is full increase code size */
                                         if (self->code_table_size == (1 << self->code_size) && 
self->code_size < LZW_CODE_MAX)
                                                 self->code_size++;
                                 }
 
+                                /* Invalid code received - just stop here */
+                                if (self->code >= self->code_table_size) {
+                                        self->last_code = self->eoi_code;
+                                        return output_length;
+                                }
+
                                 /* Convert codeword into indexes */
                                 n_written += write_indexes (self, output + n_written, output_length - 
n_written);
                         }
diff --git a/tests/test-images/fail/hang_114.gif b/tests/test-images/fail/hang_114.gif
new file mode 100644
index 000000000..4d7d252cb
Binary files /dev/null and b/tests/test-images/fail/hang_114.gif differ