Wireshark-bugs: [Wireshark-bugs] [Bug 8169] Possible infinite loop in nwmtp dissector
Comment # 2
on bug 8169
from Moshe Kaplan
(In reply to comment #1)
> (the code in question is currently at line 109 in the file)
>
> My analysis:
>
> The loop happens if (len + 12) overflows to 0, which happens when len is
> exactly (UINT32_MAX - 11). However, to even get that far requires some other
> stars to align.
>
> On systems where sizeof(gint) is 32, tvb_new_subset will see the length as
> negative and throw an exception, so it simply can't happen. On systems where
> sizeof(gint) is 64, tvb_new_subset will see the correct length. However, it
> will still throw an exception unless the packet's reported length remaining
> is at least that large.
>
> If the packet's reported length is sufficiently large then tvb_new_subset
> will pass, however the call to the mtp dissector may still throw an
> exception if insufficient real data is available.
>
> I think this means you can create this with a single small packet:
> 1. Take a regular NWMTP packet containing at least one full MTP3 payload.
> 2. Craft the 'reported length' of the packet to some value larger than
> UINT32_MAX + the original size of the packet
> 3. Craft the NWMTP length field to (UINT32_MAX - 11).
> 4. Run on a 64-bit system.
>
> Note that this requires a capture format which supports 64-bit reported
> lengths for packets. None of the modern capture formats do this (including
> pcap-ng, which still uses an unsigned 32-bit integer).
>
> ---
>
> Given all of the caveats (especially the need for a 64-bit reported length)
> I am unconvinced this is actually a realistic bug. Does my analysis match
> yours?
>
> Thanks,
> Evan
You've truly done a better job at analyzing this than I have. I'm not confident
that this is a bug, as the title (hopefully) implies. My hope is that someone
more qualified will be able to decide if it is.
Thanks for looking at it,
Moshe
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