Re: [Xen-devel] XSA-180 follow-up: repurpose xenconsoled for logging

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Jun 07, 2016 at 10:57:14AM +0100, George Dunlap wrote:
> On 06/06/16 16:48, Wei Liu wrote:
> >> A distro which isn't equipped to deal with these things is of no
> >> practical use in the real world.
> >>
> >> Any distro we care about supporting is equipped to deal with logs.
> >>
> >>>
> >>> I agree that if logging using existing logging systems were workable,
> >>> that would be a better solution.  Wei did actually spend a decent chunk
> >>> of time looking at other options, including logrotate and journald
> >>> before settling on using xenconsoled.
> >>>
> >>> A lot of this brainstorming and discussion happened off-list because
> >>> XSA-180 was still embargoed, so I can understand why it looks like this
> >>> came out of nowhere.  It would probably be good for Wei to report here
> >>> what he found and why he decided to propose this solution instead.
> >>
> >> Please do.  Until there is an understanding of why the standard
> >> mechanisms are not suitable, it is premature and naive to re-invent a 
> >> wheel.
> >>
> > 
> > With syslog and logrotate you will still end up filling up your disk.
> > Logrotate can't actively rotate log files.
> 
> FWIW CentOS 6 and 7 (which use rsyslogd and systemd-journald
> respectively) seem to have rate-limiting stuff enabled by default; the
> attached program causes a lot of CPU utilization, but no disk resource
> exhaustion.
> 

Not rate-limited on Debian Jessie installation.  We can make
recommendation that system administrators should rate-limit syslog.

But CPU utilisation is also bad, just a different kind of "bad" from
disk exhaustion.

> > You can't just tap syslog to QEMU at the moment unless you use the
> > script I sent to XSA-180 security@ discussion. That's still a hacked up
> > solution.
> > 
> > I actually don't mind having syslog deal with those, but we need to
> > provide some not-so-hacked-up way for doing it.
> 
> On Linux, it looks like you can create a socket and "connect" to /dev/log.
> 

That's the standard interface to syslog. I believe that's what openlog()
and syslog() use.  We can open a socket and write to that, but you won't
get what you want without providing the correct formated message.

Try with:
  sudo socat /dev/log STDIN

Wei.

>  -George
> 

> #include <syslog.h>
> #include <time.h>
> 
> int main(int argc, char * argv[]) {
>     int rc;
>     
>     struct timespec tv = { .tv_nsec = 5000 };
>     
>     openlog("DoS Attempt", 0, LOG_DAEMON);
> 
>     while(!(rc = nanosleep(&tv, NULL))) {
>         syslog(LOG_ERR, "This is a nasty attempt to DoS syslog\n");
>     }
> 
>     if(rc) {
>         perror("nanosleep");
>     }
> }


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