diff --git a/media/libvpx/libvpx/vp9/vp9_cx_iface.c b/media/libvpx/libvpx/vp9/vp9_cx_iface.c
index aaef05c5c8f..cab573ef9ac 100644
--- a/media/libvpx/libvpx/vp9/vp9_cx_iface.c
+++ b/media/libvpx/libvpx/vp9/vp9_cx_iface.c
@@ -8,7 +8,9 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include <assert.h>
 #include <limits.h>
+#include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
@@ -122,6 +124,7 @@ struct vpx_codec_alg_priv {
   VP9_COMP *cpi;
   unsigned char *cx_data;
   size_t cx_data_sz;
+  // pending_cx_data either is a null pointer or points into the cx_data buffer.
   unsigned char *pending_cx_data;
   size_t pending_cx_data_sz;
   int pending_frame_count;
@@ -1253,8 +1256,12 @@ static int write_superframe_index(vpx_codec_alg_priv_t *ctx) {
 
   // Write the index
   index_sz = 2 + (mag + 1) * ctx->pending_frame_count;
-  if (ctx->pending_cx_data_sz + index_sz < ctx->cx_data_sz) {
-    uint8_t *x = ctx->pending_cx_data + ctx->pending_cx_data_sz;
+  unsigned char *cx_data_end = ctx->cx_data + ctx->cx_data_sz;
+  unsigned char *pending_cx_data_end =
+      ctx->pending_cx_data + ctx->pending_cx_data_sz;
+  ptrdiff_t space_remaining = cx_data_end - pending_cx_data_end;
+  if (index_sz <= space_remaining) {
+    uint8_t *x = pending_cx_data_end;
     int i, j;
 #ifdef TEST_SUPPLEMENTAL_SUPERFRAME_DATA
     uint8_t marker_test = 0xc0;
@@ -1285,6 +1292,8 @@ static int write_superframe_index(vpx_codec_alg_priv_t *ctx) {
 #ifdef TEST_SUPPLEMENTAL_SUPERFRAME_DATA
     index_sz += index_sz_test;
 #endif
+  } else {
+    index_sz = 0;
   }
   return index_sz;
 }
@@ -1622,9 +1631,12 @@ static vpx_codec_err_t encoder_encode(vpx_codec_alg_priv_t *ctx,
               ctx->pending_frame_sizes[ctx->pending_frame_count++] = size;
             ctx->pending_frame_magnitude |= size;
             ctx->pending_cx_data_sz += size;
-            // write the superframe only for the case when
-            if (!ctx->output_cx_pkt_cb.output_cx_pkt)
+            // write the superframe only for the case when the callback function
+            // for getting per-layer packets is not registered.
+            if (!ctx->output_cx_pkt_cb.output_cx_pkt) {
               size += write_superframe_index(ctx);
+              assert(size <= cx_data_sz);
+            }
             pkt.data.frame.buf = ctx->pending_cx_data;
             pkt.data.frame.sz = ctx->pending_cx_data_sz;
             ctx->pending_cx_data = NULL;