lavf: sanity check size in av_get/append_packet().

To avoid allocating ridiculous amounts of memory for corrupted files,
read the input in chunks limited to filesize or an arbitrary large
amount when that is not known (chosen to be 50M).

libavformat/utils.c

@@ -214,38 +214,68 @@ AVInputFormat *av_find_input_format(const char *short_name)
     return NULL;
 }
 
+/* an arbitrarily chosen "sane" max packet size -- 50M */
+#define SANE_CHUNK_SIZE (50000000)
+
+/*
+ * Read the data in sane-sized chunks and append to pkt.
+ * Return the number of bytes read or an error.
+ */
+static int append_packet_chunked(AVIOContext *s, AVPacket *pkt, int size)
+{
+    int64_t chunk_size = size;
+    int orig_pos       = pkt->pos; // av_grow_packet might reset pos
+    int orig_size      = pkt->size;
+    int ret = 0;
+
+    do {
+        int prev_size = pkt->size;
+        int read_size;
+
+        /*
+         * When the caller requests a lot of data, limit it to the amount left
+         * in file or SANE_CHUNK_SIZE when it is not known
+         */
+        if (size > SANE_CHUNK_SIZE) {
+            int64_t filesize = avio_size(s) - avio_tell(s);
+            chunk_size = FFMAX(filesize, SANE_CHUNK_SIZE);
+        }
+        read_size = FFMIN(size, chunk_size);
+
+        ret = av_grow_packet(pkt, read_size);
+        if (ret < 0)
+            break;
+
+        ret = avio_read(s, pkt->data + prev_size, read_size);
+        if (ret != read_size) {
+            av_shrink_packet(pkt, prev_size + FFMAX(ret, 0));
+            break;
+        }
+
+        size -= read_size;
+    } while (size > 0);
+
+    pkt->pos = orig_pos;
+    if (!pkt->size)
+        av_free_packet(pkt);
+    return pkt->size > orig_size ? pkt->size - orig_size : ret;
+}
 
 int av_get_packet(AVIOContext *s, AVPacket *pkt, int size)
 {
-    int ret= av_new_packet(pkt, size);
+    av_init_packet(pkt);
+    pkt->data = NULL;
+    pkt->size = 0;
+    pkt->pos  = avio_tell(s);
 
-    if(ret<0)
-        return ret;
-
-    pkt->pos= avio_tell(s);
-
-    ret= avio_read(s, pkt->data, size);
-    if(ret<=0)
-        av_free_packet(pkt);
-    else
-        av_shrink_packet(pkt, ret);
-
-    return ret;
+    return append_packet_chunked(s, pkt, size);
 }
 
 int av_append_packet(AVIOContext *s, AVPacket *pkt, int size)
 {
-    int ret;
-    int old_size;
     if (!pkt->size)
         return av_get_packet(s, pkt, size);
-    old_size = pkt->size;
-    ret = av_grow_packet(pkt, size);
-    if (ret < 0)
-        return ret;
-    ret = avio_read(s, pkt->data + old_size, size);
-    av_shrink_packet(pkt, old_size + FFMAX(ret, 0));
-    return ret;
+    return append_packet_chunked(s, pkt, size);
 }