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

[J.C. Cleaver]

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?

This is limited to trends pages and I'm not sure it would be exploitable
via construction without hostsvc-add ability in xymond, but if so that's a
concern.


Thanks,
-jc