[FFmpeg-devel] [PATCH 1/3] avformat/rcwtenc: Fix potential out-of-bounds write

[Andreas Rheinhardt]

The rcwt muxer uses several counters for how much data
it has already cached: One byte counter and one counter
for how many complete blocks (of three bytes each).
These counters can become inconsistent when the muxer is
fed incomplete blocks as the muxer presumes that it is
about to write a new block at the start of each write_packet
call. E.g. sending 65535*3+1 1-byte packets (with data[0] e.g. 0x03)
will trigger an out-of-bounds write.

This patch fixes this by processing the data in complete blocks
only. This also allows to simplify the code, e.g. to remove one of
the counters.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
---
 libavformat/rcwtenc.c | 38 +++++++++++---------------------------
 1 file changed, 11 insertions(+), 27 deletions(-)

diff --git a/libavformat/rcwtenc.c b/libavformat/rcwtenc.c
index 839436ce84..d0e469ce65 100644
--- a/libavformat/rcwtenc.c
+++ b/libavformat/rcwtenc.c
@@ -65,7 +65,6 @@
 #define RCWT_BLOCK_SIZE                     3
 
 typedef struct RCWTContext {
-    int cluster_nb_blocks;
     int cluster_pos;
     int64_t cluster_pts;
     uint8_t cluster_buf[RCWT_CLUSTER_MAX_BLOCKS * RCWT_BLOCK_SIZE];
@@ -75,7 +74,6 @@ static void rcwt_init_cluster(AVFormatContext *avf)
 {
     RCWTContext *rcwt = avf->priv_data;
 
-    rcwt->cluster_nb_blocks = 0;
     rcwt->cluster_pos = 0;
     rcwt->cluster_pts = AV_NOPTS_VALUE;
     memset(rcwt->cluster_buf, 0, sizeof(rcwt->cluster_buf));
@@ -85,10 +83,10 @@ static void rcwt_flush_cluster(AVFormatContext *avf)
 {
     RCWTContext *rcwt = avf->priv_data;
 
-    if (rcwt->cluster_nb_blocks > 0) {
+    if (rcwt->cluster_pos > 0) {
         avio_wl64(avf->pb, rcwt->cluster_pts);
-        avio_wl16(avf->pb, rcwt->cluster_nb_blocks);
-        avio_write(avf->pb, rcwt->cluster_buf, (rcwt->cluster_nb_blocks * RCWT_BLOCK_SIZE));
+        avio_wl16(avf->pb, rcwt->cluster_pos / 3);
+        avio_write(avf->pb, rcwt->cluster_buf, rcwt->cluster_pos);
     }
 
     rcwt_init_cluster(avf);
@@ -129,10 +127,7 @@ static int rcwt_write_packet(AVFormatContext *avf, AVPacket *pkt)
 {
     RCWTContext *rcwt = avf->priv_data;
 
-    int in_block = 0;
-    int nb_block_bytes = 0;
-
-    if (pkt->size == 0)
+    if (pkt->size <= 2)
         return 0;
 
     /* new PTS, new cluster */
@@ -146,11 +141,11 @@ static int rcwt_write_packet(AVFormatContext *avf, AVPacket *pkt)
         return 0;
     }
 
-    for (int i = 0; i < pkt->size; i++) {
+    for (int i = 0; i < pkt->size - 2;) {
         uint8_t cc_valid;
         uint8_t cc_type;
 
-        if (rcwt->cluster_nb_blocks == RCWT_CLUSTER_MAX_BLOCKS) {
+        if (rcwt->cluster_pos == RCWT_CLUSTER_MAX_BLOCKS * RCWT_BLOCK_SIZE) {
             av_log(avf, AV_LOG_WARNING, "Starting new cluster due to size\n");
             rcwt_flush_cluster(avf);
         }
@@ -158,25 +153,14 @@ static int rcwt_write_packet(AVFormatContext *avf, AVPacket *pkt)
         cc_valid = (pkt->data[i] & 0x04) >> 2;
         cc_type = pkt->data[i] & 0x03;
 
-        if (!in_block && !(cc_valid || cc_type == 3))
-            continue;
-
-        memcpy(&rcwt->cluster_buf[rcwt->cluster_pos], &pkt->data[i], 1);
-        rcwt->cluster_pos++;
-
-        if (!in_block) {
-            in_block = 1;
-            nb_block_bytes = 1;
+        if (!(cc_valid || cc_type == 3)) {
+            i++;
             continue;
         }
 
-        nb_block_bytes++;
-
-        if (nb_block_bytes == RCWT_BLOCK_SIZE) {
-            in_block = 0;
-            nb_block_bytes = 0;
-            rcwt->cluster_nb_blocks++;
-        }
+        memcpy(&rcwt->cluster_buf[rcwt->cluster_pos], &pkt->data[i], 3);
+        rcwt->cluster_pos += 3;
+        i                 += 3;
     }
 
     return 0;
-- 
2.34.1