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Re: Buffer overflow in Nmap when using -ox - on a /8 scan
From: Daniel Miller <bonsaiviking () gmail com>
Date: Mon, 27 Jan 2014 10:35:24 -0600

On 01/26/2014 10:46 AM, Daniel Miller wrote:
On Sat, Jan 25, 2014 at 4:25 PM, Henri Doreau <henri.doreau () gmail com> wrote:
Hi,

2014-01-25 Jacek Wielemborek <d33tah () gmail com>:
Hi,

I just found a potentially interesting error. While experimenting with Nmap, I
managed to get this strange error on Nmap 6.40 from Fedora 20:

[22:46:39][/tmp][134] $ nmap localhost/8 --min-rate 100000  -ox - -sT
[...]
Huh?! What is "-sT --min-rate 100000"?

It looks like the error comes from FD_ISSET, because you forced nmap
to open sockets beyond FD_SETSIZE. What we could have is a
CHECKED_FD_ISSET, that would abort() just like CHECK_FD_SET if the
socket # is greater than FD_SETSIZE. That would make the crash a bit
nicer but wouldn't essentially change anything...

It looks like this check is supposed to fail, since it does without
optimization (search Jacek's message for "FD_SETSIZE"). So the
compiler is optimizing out the CHECKED_FD_SET check?

Dan
.

Correction: It's not an optimization difference, it's a -D_FORTIFY_SOURCE difference. In the first (optimized) case, the code runs afoul of a fortify bounds check when executing FD_ISSET, so it never gets to the checks involved in CHECKED_FD_SET on the lines below it. In the unoptimized version, -D_FORTIFY_SOURCE doesn't apply, so the code gets to Nmap's own checks and aborts there.

The cleanest solution would seem to be Henri's suggested CHECKED_FD_ISSET, but since FD_ISSET returns an int, I don't see how it could be properly wrapped in the do{}while(0) and still checked. I think an explicit check would be best, right after the assert(sd >= 0);

Dan
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