[FFmpeg-devel] [PATCH 1/2] avformat/mxfdec: Fix integer overflow in next position in mxf_read_local_tags()

[James Almer]

On 1/22/2021 7:10 PM, Michael Niedermayer wrote:
> On Fri, Jan 22, 2021 at 11:23:09AM -0300, James Almer wrote:
>> On 1/22/2021 11:09 AM, Michael Niedermayer wrote:
>>> Fixes: signed integer overflow: 9223372036854775723 + 8192 cannot be represented in type 'long'
>>> Fixes: 29072/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-4812604904177664
>>>
>>> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
>>> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
>>> ---
>>>    libavformat/mxfdec.c | 4 +++-
>>>    1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/libavformat/mxfdec.c b/libavformat/mxfdec.c
>>> index 4c932e954c..f1167761f9 100644
>>> --- a/libavformat/mxfdec.c
>>> +++ b/libavformat/mxfdec.c
>>> @@ -2865,8 +2865,10 @@ static int mxf_read_local_tags(MXFContext *mxf, KLVPacket *klv, MXFMetadataReadF
>>>            int ret;
>>>            int tag = avio_rb16(pb);
>>>            int size = avio_rb16(pb); /* KLV specified by 0x53 */
>>> -        uint64_t next = avio_tell(pb) + size;
>>> +        uint64_t next = avio_tell(pb);
>>>            UID uid = {0};
>>> +        if (next > INT64_MAX - size)
>>> +            size = 0;
>>
>> You're discarding the tag's size. The code below could misbehave as it will
>> assume it's an empty tag when it's not. Not to mention the log message will
>> print a bogus value.
>>
>> Abort instead (Invalid data or erange) in this scenario since going beyond
>> INT64_MAX is clearly not supported by avio, and then do next += size to
>> ensure the avio_seek() at the end works as expected.
> 
> I dont think this strictly implies being beyond INT64_MAX.
> but i wanted to redo this patch anyway as it contained another bug
> so next suggestion:
> 
> @@ -2865,8 +2865,11 @@ static int mxf_read_local_tags(MXFContext *mxf, KLVPacket *klv, MXFMetadataReadF
>           int ret;
>           int tag = avio_rb16(pb);
>           int size = avio_rb16(pb); /* KLV specified by 0x53 */
> -        uint64_t next = avio_tell(pb) + size;
> +        int64_t next = avio_tell(pb);
>           UID uid = {0};
> +        if (next < 0 || next > INT64_MAX - size)
> +            return next < 0 ? next : AVERROR_INVALIDDATA;
> +        next += size;
>   
>           av_log(mxf->fc, AV_LOG_TRACE, "local tag %#04x size %d\n", tag, size);
>           if (!size) { /* ignore empty tag, needed for some files with empty UMID tag */
> 
> 
> thx

That's better, yes.