Conference aosg::lsm

793.0. "Vicious User trick - write to rz device that LSM is using." by NQOS01::beast.det.dec.com::Ray Browder (Did you Exchange a walk-on part in the war for a lead role i) Fri May 09 1997 20:12

Hello,

This is a vicious user with root access or
a clumsy system administrator trick.

The idea behind this comes from a co-worker
who is researching the HP pvlinks or 
"alternate links" in their LVM product. This
is in reguard to their ability to do dual 
path to a disk with LVM.

The question is with pvlinks can you bypass
LVM and write to the block or raw device. Thus
toasting your logical volume. (I don't expect
an answer to this question.)

A companion question is, can a person with 
root access do a similar thing on a DIGITAL
UNIX system? 

We did a test on a DEC 4000 running V3.2g
of DIGITAL UNIX. I created a volume of about 
5000 blocks. I put UFS on it and mounted it.
I did a "cp -r /" to this mount point to show
that I could write to it. I checked the filesystem
and determined that there was data on that volume.

I then ran the following command:

	tar cvf /dev/rz2c /

This command ran! While this was running, I did an
ls on the mounted filesystem. It could not find any
files on the filesystem. I stopped the tar command
after a minute or so. The system did not stop the 
tar command from writhing data to the rz device that
LSM was using.

Is this a bug, a feature, or something in between? I
know that UNIX does not protect against stupidity.
Should I be able to do this? I think I remember this
being disscussed before, and that the "solution"  was
don't write to that device.

Thanks,
Ray
[email protected]

	Device drivers may be smart enough to prevent writes or
	even read/write access to a partition, when something
	else has that partition open.  But, it is much harder
	to prevent the access when the partitions aren't the
	same.  LSM doesn't usually use a whole C partition.  Since
	you did, you overwrote the data.

	It would have been no different if you had a simple UFS
	file system on something other than C and then overwrote
	C.  When you are root have you to be careful.

Thanks,

That is what I expected. It's the
old "Don't do that, you'll hurt
yourself" routine.

Thanks again.
Ray


"I know I was stupid, but it didn't 
have a warning label!" :)