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

[Michael Niedermayer <michael@niedermayer.cc>]

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On Wed, Mar 23, 2022 at 12:07:25PM +0100, Andreas Rheinhardt wrote:
> 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.

The goal of my patch was to fix the undefined behavior.
You are very correct that aiffdec has other bugs.

ill send a more complete set of fixes 

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

There will always be a question for which you do not know the correct answer.

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