Fix potential OOB read with Error Concealment

This patch fixes an OOB read when error concealment is enabled and the
partition sizes are corrupt. The partition size read from the bitstream
was not being validated in EC mode.

Change-Id: Ia81dfd4bce1ab29ee78e42320abe52cee8318974

1 files changed, 35 insertions, 10 deletions

diff --git a/vp8/decoder/decodframe.c b/vp8/decoder/decodframe.c
index 9b35ffd66..5ddf09567 100644
--- a/vp8/decoder/decodframe.c
+++ b/vp8/decoder/decodframe.c
@@ -495,6 +495,15 @@ static void setup_token_decoder_partition_input(VP8D_COMP *pbi)
 #endif
 }
 
+
+static int read_is_valid(const unsigned char *start,
+                         size_t               len,
+                         const unsigned char *end)
+{
+    return (start + len > start && start + len <= end);
+}
+
+
 static void setup_token_decoder(VP8D_COMP *pbi,
                                 const unsigned char *cx_data)
 {
@@ -529,26 +538,42 @@ static void setup_token_decoder(VP8D_COMP *pbi,
     for (i = 0; i < num_part; i++)
     {
         const unsigned char *partition_size_ptr = cx_data + i * 3;
-        ptrdiff_t            partition_size;
+        ptrdiff_t            partition_size, bytes_left;
+
+        bytes_left = user_data_end - partition;
 
         /* Calculate the length of this partition. The last partition
-         * size is implicit.
+         * size is implicit. If the partition size can't be read, then
+         * either use the remaining data in the buffer (for EC mode)
+         * or throw an error.
          */
         if (i < num_part - 1)
         {
-            partition_size = read_partition_size(partition_size_ptr);
+            if (read_is_valid(partition_size_ptr, 3, user_data_end))
+                partition_size = read_partition_size(partition_size_ptr);
+            else if(pbi->ec_enabled)
+                partition_size = bytes_left;
+            else
+                vpx_internal_error(&pc->error, VPX_CODEC_CORRUPT_FRAME,
+                                   "Truncated partition size data");
         }
         else
+            partition_size = bytes_left;
+
+        /* Validate the calculated partition length. If the buffer
+         * described by the partition can't be fully read, then restrict
+         * it to the portion that can be (for EC mode) or throw an error.
+         */
+        if (!read_is_valid(partition, partition_size, user_data_end))
         {
-            partition_size = user_data_end - partition;
+            if(pbi->ec_enabled)
+                partition_size = bytes_left;
+            else
+                vpx_internal_error(&pc->error, VPX_CODEC_CORRUPT_FRAME,
+                                   "Truncated packet or corrupt partition "
+                                   "%d length", i + 1);
         }
 
-        if (!pbi->ec_enabled && (partition + partition_size > user_data_end
-            || partition + partition_size < partition))
-            vpx_internal_error(&pc->error, VPX_CODEC_CORRUPT_FRAME,
-                               "Truncated packet or corrupt partition "
-                               "%d length", i + 1);
-
         if (vp8dx_start_decode(bool_decoder, partition, partition_size))
             vpx_internal_error(&pc->error, VPX_CODEC_MEM_ERROR,
                                "Failed to allocate bool decoder %d", i + 1);