[Xymon] Possible bug in xymon svcstatus causing segfault (buffer overrun?)

[Jeremy Laidman]

Ugh, just saw your reply in my spam folder!

On Fri, 1 Sept 2023 at 23:40, J.C. Cleaver <cleaver at terabithia.org> wrote:

>
> On Tue, August 29, 2023 23:40, Jeremy Laidman wrote:
>
> >
> > Elsewhere in other code, the SBUF_REALLOC macro is used to add extra
> > bufferspace by calculating a new buffer size of [existing buffer size
> plus
> > 4k plus the length of the string being added]. So if the string to be
> > added
> > had 20 bytes, SBUF_REALLOC would be used to add 4k+20 bytes to the buffer
> > size. Notably, the size of the string is included in the calculation. For
> > example this bit of code elsewhere in the same file does a similar buffer
> > extension:
> >
> >                         if ((strlen(rrdlink) + strlen(partlink) + 1) >=
> > rrdlink_buflen) {
> >                                 SBUF_REALLOC(rrdlink, rrdlink_buflen +
> > strlen(partlink) + 4096);
> >                         }
> >                         strncat(rrdlink, partlink, (rrdlink_buflen -
> > strlen(rrdlink)));
> >
> > Note the extra "+ strlen(<stringvar>)" bit in the extra memory being
> > allocated that isn't there in the first block of code above. So my
> > thinking
> > is that the SBUF_REALLOC() needs to take the string's size into account,
> > like so:
> >
> >                                 SBUF_REALLOC(allrrdlinks,
> > allrrdlinks_buflen+strlen(onelink)+4096);
> >
> > Is anyone with C skills able to confirm that this does indeed look like a
> > bug, and also that my proposed change is a suitable fix? After applying
> > this change and recompiling, we haven't yet seen a seg fault from this
> bit
> > of code, so I'm hoping it's all fixed, and without negative side effects.
> >
> > Cheers
> > Jeremy
>
> Based on my looking over it, I would confirm that this seems to be a bug.
> Specifically, it looks like we missed this part of the fix in
> https://sourceforge.net/p/xymon/code/8069/?page=7#diff-1 while trying to
> get ahead of possible buffer overflows for various CVE's
> (https://nvd.nist.gov/vuln/detail/CVE-2019-13486, I suppose). I can't see
> a reason not to add the length in here without a guard somewhere else for
> the length.
>
> Furthermore, other uses of this macro pretty much all include the length
> of the incoming string or limit the concat to a specific size, which
> obviously isn't ideal for URLs here.
>
> If you have a reproducer and your patch fixes it, I suspect that will be
> sufficient and we can add it in. Can you send a coredump from a crash off
> list by any chance (using the debuginfo RPMs) just to double-check?
>

Sorry, can you define "reproducer"? Is that a coding term I haven't yet
come across?

I have core dumps, but I'll see about using the debuginfo RPMs (assuming
they're on Terabithia) and get a core dump from an install of that.

Thanks
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