Re: [FFmpeg-devel] [PATCH 2/2] avformat/aiffdec: check allocated size for overflow

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Michael Niedermayer:
> Fixes: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'
> Fixes: 45891/clusterfuzz-testcase-minimized-ffmpeg_dem_AIFF_fuzzer-6159183893889024
> 
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>  libavformat/aiffdec.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/libavformat/aiffdec.c b/libavformat/aiffdec.c
> index 3634bb4960..c2b8e0dede 100644
> --- a/libavformat/aiffdec.c
> +++ b/libavformat/aiffdec.c
> @@ -72,7 +72,12 @@ static int get_tag(AVIOContext *pb, uint32_t * tag)
>  /* Metadata string read */
>  static void get_meta(AVFormatContext *s, const char *key, int size)
>  {
> -    uint8_t *str = av_malloc(size+1);
> +    uint8_t *str;
> +
> +    if (size == INT_MAX)
> +        return;
> +
> +    str = av_malloc(size+1);
>  
>      if (str) {
>          int res = avio_read(s->pb, str, size);

If a size of INT_MAX is legal, then you can just use "size + 1U" to
avoid the wraparound. (The allocation will then fail with the default
value of max_alloc_size and in this case the avio_skip() will be
executed, so that we don't lose sync (your patch does that).)
Looking at get_tag() shows that the size actually comes from a uint32_t
which gets truncated to INT_MAX if it is not representable in an int. I
don't know whether the specs mandate this behaviour (probably not, so
one loses sync). If one wanted to impose an arbitrary limit, I will use
something much smaller than INT_MAX.

- Andreas
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