avformat/rtmppkt: Convert ff_amf_get_field_value() to bytestream2

Fixes: out of array accesses

Found-by: JunDong Xie of Ant-financial Light-Year Security Lab
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit ffcc8221)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rtmppkt.c

@@ -505,53 +505,70 @@ int ff_amf_tag_size(const uint8_t *data, const uint8_t *data_end)
     return bytestream2_tell(&gb);
 }
 
-int ff_amf_get_field_value(const uint8_t *data, const uint8_t *data_end,
+static int amf_get_field_value2(GetByteContext *gb,
                            const uint8_t *name, uint8_t *dst, int dst_size)
 {
     int namelen = strlen(name);
     int len;
 
-    while (*data != AMF_DATA_TYPE_OBJECT && data < data_end) {
-        len = ff_amf_tag_size(data, data_end);
-        if (len < 0)
-            len = data_end - data;
-        data += len;
+    while (bytestream2_peek_byte(gb) != AMF_DATA_TYPE_OBJECT && bytestream2_get_bytes_left(gb) > 0) {
+        int ret = amf_tag_skip(gb);
+        if (ret < 0)
+            return -1;
     }
-    if (data_end - data < 3)
+    if (bytestream2_get_bytes_left(gb) < 3)
         return -1;
-    data++;
+    bytestream2_get_byte(gb);
+
     for (;;) {
-        int size = bytestream_get_be16(&data);
+        int size = bytestream2_get_be16(gb);
         if (!size)
             break;
-        if (size < 0 || size >= data_end - data)
+        if (size < 0 || size >= bytestream2_get_bytes_left(gb))
             return -1;
-        data += size;
-        if (size == namelen && !memcmp(data-size, name, namelen)) {
-            switch (*data++) {
+        bytestream2_skip(gb, size);
+        if (size == namelen && !memcmp(gb->buffer-size, name, namelen)) {
+            switch (bytestream2_get_byte(gb)) {
             case AMF_DATA_TYPE_NUMBER:
-                snprintf(dst, dst_size, "%g", av_int2double(AV_RB64(data)));
+                snprintf(dst, dst_size, "%g", av_int2double(bytestream2_get_be64(gb)));
                 break;
             case AMF_DATA_TYPE_BOOL:
-                snprintf(dst, dst_size, "%s", *data ? "true" : "false");
+                snprintf(dst, dst_size, "%s", bytestream2_get_byte(gb) ? "true" : "false");
                 break;
             case AMF_DATA_TYPE_STRING:
-                len = bytestream_get_be16(&data);
-                av_strlcpy(dst, data, FFMIN(len+1, dst_size));
+                len = bytestream2_get_be16(gb);
+                if (dst_size < 1)
+                    return -1;
+                if (dst_size < len + 1)
+                    len = dst_size - 1;
+                bytestream2_get_buffer(gb, dst, len);
+                dst[len] = 0;
                 break;
             default:
                 return -1;
             }
             return 0;
         }
-        len = ff_amf_tag_size(data, data_end);
-        if (len < 0 || len >= data_end - data)
+        len = amf_tag_skip(gb);
+        if (len < 0 || bytestream2_get_bytes_left(gb) <= 0)
             return -1;
-        data += len;
     }
     return -1;
 }
 
+int ff_amf_get_field_value(const uint8_t *data, const uint8_t *data_end,
+                           const uint8_t *name, uint8_t *dst, int dst_size)
+{
+    GetByteContext gb;
+
+    if (data >= data_end)
+        return -1;
+
+    bytestream2_init(&gb, data, data_end - data);
+
+    return amf_get_field_value2(&gb, name, dst, dst_size);
+}
+
 static const char* rtmp_packet_type(int type)
 {
     switch (type) {