Re: Babel: prevent an infinite loop while parsing sub-TLV

[Pascal Quantin <pascal () wireshark org>]

Le ven. 18 oct. 2019 à 20:58, Pascal Quantin <pascal () wireshark org> a
écrit :

> Hi Juliusz,
>
> Le ven. 18 oct. 2019 à 20:51, Juliusz Chroboczek <jch () irif fr> a écrit :
>
> > Dear Pascal,
> >
> > I've just seen your commit dd15b2, which I believe is incorrect.
> >
> >
> > https://code.wireshark.org/review/gitweb?p=wireshark.git;a=commit;h=dd15b203c7ec8a8ec5c930cf018c838991ee3182
> >
> > In dissect_babel_subtlvs, sublen is the length of the TLV body; the full
> > TLV is of length sublen+2.  At the end of the loop, the code is correct:
> >
> >         beg += (sublen+2);
> >
> > Hence, it is perfectly fine to have sublen=0.  Unless I'm wrong, your
> > commit is incorrect.
>
> Maybe, honestly I do not remember the details as it was done a few months
> ago. It would require me to dive in once I have the time (so not now).
>
>
> > Could you please either explain why I'm wrong, or else revert commit
> > dd15b2?
>
> I will not revert this commit as it solved/workarounded an infinite loop,
> as seen in https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=15856
> But is you can come up with a better solution that ensures that we do not
> have the infinite loop while still being correct, you are welcome and I
> will try to review it

Without even looking at the capture from bug 15856, I can see an infinite
loop when subtype = MESSAGE_SUB_PAD1 and sublen = 0:
        if(subtype == MESSAGE_SUB_PAD1){
            beg += sublen;
            continue;
        }
Maybe the test for sublen value should be put only when the subtype is
MESSAGE_SUB_PAD1? Or when subtype is different from MESSAGE_SUB_PADN?
As you seem to know this protocol, your feedback is welcome.

Best regards,
Pascal.

>
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