[nginx] Mp4: fixed buffer underread while updating stsz atom.

[Sergey Kandaurov]

details:   https://hg.nginx.org/nginx/rev/f88c253ac8b3
branches:  
changeset: 9276:f88c253ac8b3
user:      Roman Arutyunyan <arut at nginx.com>
date:      Mon Aug 12 18:20:43 2024 +0400
description:
Mp4: fixed buffer underread while updating stsz atom.

While cropping an stsc atom in ngx_http_mp4_crop_stsc_data(), a 32-bit integer
overflow could happen, which could result in incorrect seeking and a very large
value stored in "samples".  This resulted in a large invalid value of
trak->end_chunk_samples.  This value is further used to calculate the value of
trak->end_chunk_samples_size in ngx_http_mp4_update_stsz_atom().  While doing
this, a large invalid value of trak->end_chunk_samples could result in reading
memory before stsz atom start.  This could potentially result in a segfault.

diffstat:

 src/http/modules/ngx_http_mp4_module.c |  7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diffs (31 lines):

diff -r b5550a7f16c7 -r f88c253ac8b3 src/http/modules/ngx_http_mp4_module.c
--- a/src/http/modules/ngx_http_mp4_module.c	Fri Aug 09 19:12:26 2024 +0400
+++ b/src/http/modules/ngx_http_mp4_module.c	Mon Aug 12 18:20:43 2024 +0400
@@ -3099,7 +3099,8 @@ static ngx_int_t
 ngx_http_mp4_crop_stsc_data(ngx_http_mp4_file_t *mp4,
     ngx_http_mp4_trak_t *trak, ngx_uint_t start)
 {
-    uint32_t               start_sample, chunk, samples, id, next_chunk, n,
+    uint64_t               n;
+    uint32_t               start_sample, chunk, samples, id, next_chunk,
                            prev_samples;
     ngx_buf_t             *data, *buf;
     ngx_uint_t             entries, target_chunk, chunk_samples;
@@ -3160,7 +3161,7 @@ ngx_http_mp4_crop_stsc_data(ngx_http_mp4
                        "samples:%uD, id:%uD",
                        start_sample, chunk, next_chunk - chunk, samples, id);
 
-        n = (next_chunk - chunk) * samples;
+        n = (uint64_t) (next_chunk - chunk) * samples;
 
         if (start_sample < n) {
             goto found;
@@ -3182,7 +3183,7 @@ ngx_http_mp4_crop_stsc_data(ngx_http_mp4
                    "sample:%uD, chunk:%uD, chunks:%uD, samples:%uD",
                    start_sample, chunk, next_chunk - chunk, samples);
 
-    n = (next_chunk - chunk) * samples;
+    n = (uint64_t) (next_chunk - chunk) * samples;
 
     if (start_sample > n) {
         ngx_log_error(NGX_LOG_ERR, mp4->file.log, 0,