Mp4: fixed possible chunk offset overflow.

In "co64" atom chunk start offset is a 64-bit unsigned integer.  When trimming
the "mdat" atom, chunk offsets are casted to off_t values which are typically
64-bit signed integers.  A specially crafted mp4 file with huge chunk offsets
may lead to off_t overflow and result in negative trim boundaries.

The consequences of the overflow are:
- Incorrect Content-Length header value in the response.
- Negative left boundary of the response file buffer holding the trimmed "mdat".
  This leads to pread()/sendfile() errors followed by closing the client
  connection.

On rare systems where off_t is a 32-bit integer, this scenario is also feasible
with the "stco" atom.

The fix is to add checks which make sure data chunks referenced by each track
are within the mp4 file boundaries.  Additionally a few more checks are added to
ensure mp4 file consistency and log errors.

1 files changed, 64 insertions, 11 deletions

diff --git a/src/http/modules/ngx_http_mp4_module.c b/src/http/modules/ngx_http_mp4_module.c
index 618bf787b..c1006abbb 100644
--- a/src/http/modules/ngx_http_mp4_module.c
+++ b/src/http/modules/ngx_http_mp4_module.c
@@ -3116,6 +3116,13 @@ ngx_http_mp4_update_stsz_atom(ngx_http_mp4_file_t *mp4,
                        "chunk samples sizes:%uL",
                        trak->start_chunk_samples_size);
 
+        if (trak->start_chunk_samples_size > (uint64_t) mp4->end) {
+            ngx_log_error(NGX_LOG_ERR, mp4->file.log, 0,
+                          "too large mp4 start samples size in \"%s\"",
+                          mp4->file.name.data);
+            return NGX_ERROR;
+        }
+
         if (mp4->length) {
             if (trak->end_sample - trak->start_sample > entries) {
                 ngx_log_error(NGX_LOG_ERR, mp4->file.log, 0,
@@ -3135,6 +3142,13 @@ ngx_http_mp4_update_stsz_atom(ngx_http_mp4_file_t *mp4,
             ngx_log_debug1(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
                            "mp4 stsz end_chunk_samples_size:%uL",
                            trak->end_chunk_samples_size);
+
+            if (trak->end_chunk_samples_size > (uint64_t) mp4->end) {
+                ngx_log_error(NGX_LOG_ERR, mp4->file.log, 0,
+                              "too large mp4 end samples size in \"%s\"",
+                              mp4->file.name.data);
+                return NGX_ERROR;
+            }
         }
 
         atom_size = sizeof(ngx_mp4_stsz_atom_t) + (data->last - data->pos);
@@ -3226,6 +3240,7 @@ ngx_http_mp4_update_stco_atom(ngx_http_mp4_file_t *mp4,
 {
     size_t                atom_size;
     uint32_t              entries;
+    uint64_t              chunk_offset, samples_size;
     ngx_buf_t            *atom, *data;
     ngx_mp4_stco_atom_t  *stco_atom;
 
@@ -3256,8 +3271,19 @@ ngx_http_mp4_update_stco_atom(ngx_http_mp4_file_t *mp4,
 
     data->pos += trak->start_chunk * sizeof(uint32_t);
 
-    trak->start_offset = ngx_mp4_get_32value(data->pos);
-    trak->start_offset += trak->start_chunk_samples_size;
+    chunk_offset = ngx_mp4_get_32value(data->pos);
+    samples_size = trak->start_chunk_samples_size;
+
+    if (chunk_offset > (uint64_t) mp4->end - samples_size
+        || chunk_offset + samples_size > NGX_MAX_UINT32_VALUE)
+    {
+        ngx_log_error(NGX_LOG_ERR, mp4->file.log, 0,
+                      "too large chunk offset in \"%s\"",
+                      mp4->file.name.data);
+        return NGX_ERROR;
+    }
+
+    trak->start_offset = chunk_offset + samples_size;
     ngx_mp4_set_32value(data->pos, trak->start_offset);
 
     ngx_log_debug1(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
@@ -3276,9 +3302,19 @@ ngx_http_mp4_update_stco_atom(ngx_http_mp4_file_t *mp4,
         data->last = data->pos + entries * sizeof(uint32_t);
 
         if (entries) {
-            trak->end_offset =
-                            ngx_mp4_get_32value(data->last - sizeof(uint32_t));
-            trak->end_offset += trak->end_chunk_samples_size;
+            chunk_offset = ngx_mp4_get_32value(data->last - sizeof(uint32_t));
+            samples_size = trak->end_chunk_samples_size;
+
+            if (chunk_offset > (uint64_t) mp4->end - samples_size
+                || chunk_offset + samples_size > NGX_MAX_UINT32_VALUE)
+            {
+                ngx_log_error(NGX_LOG_ERR, mp4->file.log, 0,
+                              "too large chunk offset in \"%s\"",
+                              mp4->file.name.data);
+                return NGX_ERROR;
+            }
+
+            trak->end_offset = chunk_offset + samples_size;
 
             ngx_log_debug1(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
                            "end chunk offset:%O", trak->end_offset);
@@ -3409,7 +3445,7 @@ ngx_http_mp4_update_co64_atom(ngx_http_mp4_file_t *mp4,
     ngx_http_mp4_trak_t *trak)
 {
     size_t                atom_size;
-    uint64_t              entries;
+    uint64_t              entries, chunk_offset, samples_size;
     ngx_buf_t            *atom, *data;
     ngx_mp4_co64_atom_t  *co64_atom;
 
@@ -3440,8 +3476,17 @@ ngx_http_mp4_update_co64_atom(ngx_http_mp4_file_t *mp4,
 
     data->pos += trak->start_chunk * sizeof(uint64_t);
 
-    trak->start_offset = ngx_mp4_get_64value(data->pos);
-    trak->start_offset += trak->start_chunk_samples_size;
+    chunk_offset = ngx_mp4_get_64value(data->pos);
+    samples_size = trak->start_chunk_samples_size;
+
+    if (chunk_offset > (uint64_t) mp4->end - samples_size) {
+        ngx_log_error(NGX_LOG_ERR, mp4->file.log, 0,
+                      "too large chunk offset in \"%s\"",
+                      mp4->file.name.data);
+        return NGX_ERROR;
+    }
+
+    trak->start_offset = chunk_offset + samples_size;
     ngx_mp4_set_64value(data->pos, trak->start_offset);
 
     ngx_log_debug1(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
@@ -3460,9 +3505,17 @@ ngx_http_mp4_update_co64_atom(ngx_http_mp4_file_t *mp4,
         data->last = data->pos + entries * sizeof(uint64_t);
 
         if (entries) {
-            trak->end_offset =
-                            ngx_mp4_get_64value(data->last - sizeof(uint64_t));
-            trak->end_offset += trak->end_chunk_samples_size;
+            chunk_offset = ngx_mp4_get_64value(data->last - sizeof(uint64_t));
+            samples_size = trak->end_chunk_samples_size;
+
+            if (chunk_offset > (uint64_t) mp4->end - samples_size) {
+                ngx_log_error(NGX_LOG_ERR, mp4->file.log, 0,
+                              "too large chunk offset in \"%s\"",
+                              mp4->file.name.data);
+                return NGX_ERROR;
+            }
+
+            trak->end_offset = chunk_offset + samples_size;
 
             ngx_log_debug1(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
                            "end chunk offset:%O", trak->end_offset);