Re: [FFmpeg-devel] [PATCH] avformat/mov: abort reading truncated stts

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

Gyan Doshi:
> 
> 
> On 2021-12-21 01:27 am, Andreas Rheinhardt wrote:
>> Gyan Doshi:
>>> Avoids overreading the box and ingesting absurd values into stts_data
>>> ---
>>>
>>> Fixes prolonged demuxing for fuzzer-generated files in the loop added in
>>> patch for max_stts_delta
>>>
>>>   libavformat/mov.c | 5 +++++
>>>   1 file changed, 5 insertions(+)
>>>
>>> diff --git a/libavformat/mov.c b/libavformat/mov.c
>>> index 2aed6e80ef..8d88119b29 100644
>>> --- a/libavformat/mov.c
>>> +++ b/libavformat/mov.c
>>> @@ -2935,6 +2935,11 @@ static int mov_read_stts(MOVContext *c,
>>> AVIOContext *pb, MOVAtom atom)
>>>       avio_rb24(pb); /* flags */
>>>       entries = avio_rb32(pb);
>>>   +    if (atom.size < 8 + entries*8) {
>> This can overflow.
> 
> Can you illustrate?
> 
> atom.size is int64; entries is uint32.
> 
> And cppreference says,
> 
> "If the signed type can represent all values of the unsigned type, then
> the operand with the unsigned type is implicitly converted to the signed
> type. "
> 

8 + entries * 8 is calculated using unsigned with potential (defined)
wraparound; only afterwards is the result converted to int64_t
(presuming you have 32bit unsigned as usual) for the comparison.

- Andreas
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