[CentOS] xen and hwclock

R P Herrold herrold at centos.org
Thu Apr 23 22:11:15 UTC 2009


On Thu, 23 Apr 2009, Ugo Bellavance wrote:

> Mintairov Mihail a écrit :
>> Hello All, I have a problem with hwclock on centos 5.3. in xen guest
>> environment.

>> # hwclock --debug
>> hwclock from util-linux-2.13-pre7
>> hwclock: Open of /dev/rtc failed, errno=19: No such device.
>> No usable clock interface found.
>> Cannot access the Hardware Clock via any known method.

yes - this also shows up in the dmesg at boot time in the 
domU.  I noted it when doing LSB testing, as part of their 
3.2 suite wanted to tickle the hardware clock.

>> So, I'm afraid that I can not change my time by ntp.
>>
>> In CentOS mailing list I found a thread "hwclock and util-linux on 5.3",
>> but there is still no answer and discussion stopped. Maybe someone knows
>> the solution of this problem.
>
> Maybe the clock is sync'd with the host?

essentially this is the case -- the host (the dom0) has 
exclusive write access as to the clock.  -- There is 
intentionally should be no solution to permit a domU to alter 
the hwclock, as a domU should not be able to affect the domO's 
clock.

To permit a domU to do do so would permit the client to 
control the master outer ring zero's crontab, atd, and such

Kerberos attacks [keeping a ticket in force longer than 
intended], and more also come to mind, if a arbitrary domU 
could control the clock

- Russ herrold


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