Re: [FFmpeg-devel] [PATCH] libavformat/dashdec: Fix buffer overflow in segment URL resolution

[Michael Niedermayer <michael@niedermayer.cc>]

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Hi

On Fri, Apr 11, 2025 at 03:48:08PM +0800, xiaohuanshu@gmail.com wrote:
> From: xiaohuanshu <xiaohuanshu@gmail.com>
> 
> Problem:
> The max_url_size calculation for DASH segment URLs only considered the base URL
> length, leading to buffer overflow when the segment's sourceURL exceeded the
> pre-allocated buffer. This triggered the log error:
> "DASH request for url 'invalid:truncated'".
> 
> Reproduce:
> 1. A test sample "long-sourceurl-sample.mpd" (deliberately designed with a long
>    sourceURL) was uploaded to VideoLAN's repository.
> 2. Reproduce with short base path:
>    ffmpeg -i /tmp/short_path/long-sourceurl-sample.mpd
>    -> Triggers "invalid:truncated" error
> 3. With artificially lengthened base path (e.g. /aaa/../bbb/../...):
>    ffmpeg -i /long/../path/../with/../many/../segments/long-sourceurl-sample.mpd
>    -> URL resolves correctly (though HTTP fetch fails due to fake URL)
> 
> Fix:
> Recalculate max_url_size by considering both base URL and sourceURL lengths,
> ensuring sufficient buffer allocation during URL concatenation.
> 
> Signed-off-by: xiaohuanshu <xiaohuanshu@gmail.com>
> ---
>  libavformat/dashdec.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/libavformat/dashdec.c b/libavformat/dashdec.c
> index c3f3d7f3f8..f604d363c4 100644
> --- a/libavformat/dashdec.c
> +++ b/libavformat/dashdec.c
> @@ -606,7 +606,7 @@ static int parse_manifest_segmenturlnode(AVFormatContext *s, struct representati
>      char *initialization_val = NULL;
>      char *media_val = NULL;
>      char *range_val = NULL;
> -    int max_url_size = c ? c->max_url_size: MAX_URL_SIZE;
> +    int max_url_size = 0;
>      int err;
>  
>      if (!av_strcasecmp(fragmenturl_node->name, "Initialization")) {
> @@ -620,6 +620,12 @@ static int parse_manifest_segmenturlnode(AVFormatContext *s, struct representati
>                  xmlFree(initialization_val);
>                  return AVERROR(ENOMEM);
>              }
> +            max_url_size = FFMAX(

> +                c ? c->max_url_size : 0,

how can c be NULL here ?


> +                initialization_val ? aligned(strlen(initialization_val) +
> +                                             (rep_id_val ? strlen(rep_id_val) : 0) +
> +                                             (rep_bandwidth_val ? strlen(rep_bandwidth_val) : 0)) : 0);
> +            max_url_size = max_url_size ? max_url_size : MAX_URL_SIZE;
>              rep->init_section->url = get_content_url(baseurl_nodes, 4,
>                                                       max_url_size,
>                                                       rep_id_val,
> @@ -641,6 +647,12 @@ static int parse_manifest_segmenturlnode(AVFormatContext *s, struct representati
>                  xmlFree(media_val);
>                  return AVERROR(ENOMEM);
>              }
> +            max_url_size = FFMAX(
> +                c ? c->max_url_size : 0,

> +                initialization_val ? aligned(strlen(initialization_val) +

how can initialization_val be non NULL here ?

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I have often repented speaking, but never of holding my tongue.
-- Xenocrates

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