[Xen-devel] xend crash/hang scenario

[Mike Culbertson <mike@xxxxxxxxxxxx>]

I inadvertently found a way to crash/hang xend and almost completely lock 
oneself out of a domain.  I was testing a new domain, and without thinking I 
did 'cat /dev/urandom' (rather than head or tail, oopsie) to make sure it was 
functioning. The resulting flood of output pegged xend at 99% cpu, where it 
stayed for 10 minutes before I killed it off.  The real issue came after a 
restart, when I attempted to console back into the running domain. Either I 
had not managed to ctrl+c the `cat` prior to hanging xend or the output 
buffer was so full that it continued to spew garbage at me when I would 
restart xend and console in (I'm guessing).  I got back on the the console a 
minute or so later by either managing to ctrl+c at the right time, or the 
output buffer finally emptied.

This specific situation is not so scary since only people who didn't sleep 
enough *cough* would be cat-ing /dev/urandom, but I found that the event is 
not limited to something that silly.  It seems that any large output of 
binary to the console will hang xend. I can reproduce the exact situation 
with "cat /usr/bin/vim" or any other decent-sized binary.  Xend managed to 
recover from cat-ing binaries up to about 1.5mb, but anything larger than 2mb 
had xend using 99% cpu anywhere from 5 minutes to indefinitely (longer than I 
could wait anyway). Memory usage during the hangs does not go up appreciably.  
Strategic ctrl+c gets me back to console with relative ease, but it still 
requires a xend restart (sometimes 2 or 3) before I can get back in.

In contrast to the binary output, even extremely large text files (500mb+) 
appear to have no ill effect, even when cat'd in a while loop.

I have not tried outside of my setup so I don't know if it's reproducable, but 
it is very concerning to me that any user in any domain could, with a typo, 
crush the CPU and take out all management capability as well.  I'll provide 
any additional info if it will help.

Running Xen 2.0.3
Hardware: Dual Athlon XP 1900+ (1.6g) 512mb RAM
Domain0: Gentoo 2004.3 running 2.6.10-xen0 128mb RAM allocated CPU0
DomainU: Gentoo 2004.3 2.6.10-xenU / default config / 128mb RAM allocated CPU1

-- 
+--------------------------------+
Mike Culbertson
Senior Systems Administrator
DiD : 646-230-8752/(M)646-382-4779
Mail: mike@xxxxxxxxxxxx
+--------------------------------+


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