Re: [gentoo-dev] Signed push & clock drift rejection

[Patrick McLean <chutzpah@gentoo.org>]

On Tue, 19 Jul 2016 21:23:16 +0300
Andrew Savchenko <bircoph@gentoo.org> wrote:
> On Mon, 18 Jul 2016 22:21:22 -0400 waltdnes@waltdnes.org wrote:
> > On Mon, Jul 18, 2016 at 11:27:09PM +0300, Andrew Savchenko wrote  
> > > 
> > > As I wrote earlier in this thread, ntp server is not a guarantee
> > > that such problems will not happen. If hardware clocked was
> > > significantly offset during boot, it may take several _hours_ for
> > > ntp to fix this via clock skew. Apparantly commit may be made
> > > during these several hours.  
> > 
> >   I'm amazed that "robust linux servers" are deathly afraid of
> > simply setting the time, and being done with it. And while we're at
> > it, if a developer is doing development on a server machine, he may
> > have other problems to worry about.  At home I occasionally
> > manually run a script that includes the 2 lines...  
> 
> I never said anything about "robust linux servers". If you think
> than only servers need a gradual clock slew instead of stepping,
> you are mistaken.
> 
> > /usr/bin/sudo /usr/bin/openrdate -n -s ca.pool.ntp.org
> > /usr/bin/sudo /sbin/hwclock --systohc  
> 
> And if time delta is significant, the system may become broken
> this way. Congratulations.

Really? I have many times skewed time by weeks using ntpdate with no
issues.

> The gradual NTP time slew was not invented because people were lazy
> to run two simple commands. Actually it is just the opposite:
> setting system time via a single huge step is much easier to
> implement than a proper adjustment via a series of small time slews.
> For servers it is indeed important in many ways, including
> time stamp based cryptography as kerberos or database integrity.

Sure randomly skewing time can cause weird issues, which is why it is a
manual operation (and/or boot time operation).

> 
> But desktops also do need a proper time adjustment:
> - Filesystems will not operate correctly with time stamps in
> future, in the best keys they will be marked/reported as needed a
> repair procedure.

I have only ever seen ext4 complain about the superblock timestamp,
and fsck generally just corrects without issue, and it generally is
only an issue after a reboot.

> - Cron jobs may go broken too as chithanh mentioned already.

Get a better crond? Decent cron daemons can detect time skews and act
accordingly.

> - Video encoding is not happy with time shifts at all. While small
> predictable slews can be tolerated, large jumps will just broke the
> process.

Anything that cares about having a monotonic clock, and doesn't
actually care about the real time (like video encoding) should just use
the monotonic clock, not the system time.

> - System may become *vulnerable* because of time stamp based attack.
> Though it is not easy to use such behaviour, it is still possible.

Once again, monotonic clock exists for a reason. If you want to
talk about vulnerabilities, you do realize that doesn't work properly
unless the client and server have reasonably similar system times.

> - and many more...
> 
> Best regards,
> Andrew Savchenko