[vox-tech] Access change, possible virus problem

[Ken Bloom]

On Fri, 04 Jan 2008 09:51:28 -0800
Jeff Newmiller <jdnewmil at dcn.davis.ca.us> wrote:

(reorganizing the quoting a bit to make it fit the logical sequence of
explanation)

> Steve Weiss wrote:
> > Last week my Ubuntu 7.10 system started denying me the right to
> > create or delete directories in a separate data partition created
> > when we setup my dual-boot (WinXP2) system at an installfest last
> > October. I'd had no trouble doing this before last week. I can
> > create and delete files within the directories, though.

> > Here's the same for the partition itself, named /media/Data/mydata:
> > steve at SteveW:~$ ls -l /media/Data
> > total 40
> > -rwxrwx---  1 root plugdev 8192 1980-01-01 00:00 fsck0000.rec
> > -rwxrwx---  1 root plugdev 8192 1980-01-01 00:00 fsck0001.rec
> > dr-xr-x--- 42 root plugdev 8192 2007-10-06 23:58 mydata
> > drwxrwx---  2 root plugdev 8192 2007-10-06 22:38 Recycled
> > -rwxrwx---  1 root plugdev 8192 2007-12-30 00:29 vsnap.idx

> I note that your "mydata" directory has no write access enabled.
> This will prevent you from creating any files or directories
> immediately within "mydata" (though subdirectories below that will
> not be affected).
> 
> Creating a file in a directory depends on the execute and
> write permissions being enabled for you on that directory...
> permissions which are visible when you list them from the directory
> that contains the directory you want to modify (one level up from
> where you want to create files).

Because vfat filesystems contain a read-only flag, which is implemented
by DOS/Windows, the kernel developers chose to expose this flag in
Linux as POSIX permissions. It's the write permission, and it is
possible to turn that on and off using  chmod +w or chmod -w. My guess
is that the partition is vfat, and the read-only flag is set just about
everywhere that you'd be interested in reading and writing on the
partition. chmod --recursive +w should fix things.

> > One suspicious event occurred when I tried to copy data from one of
> > my kid's CDs to a flash drive. Turns out the CD had multiple
> > viruses on it. Ubuntu crashed several times just copying the files,
> > while other times the flash drive would refuse to accept any more
> > files although there was plenty of room on it, and it would unmount
> > itself. I later booted into Windows and scanned everything for
> > viruses. It only found them on the CD, not on either the Windows or
> > data partition, and not on the flash drive. (Of course, it couldn't
> > see the Linux partition.)

When copying off a CD, the resulting copy will be write-protected.
This is because you can't write to CD's in the ordinary way, so the
files don't have write permission to begin with. When they are copied,
the permissions are preserved as best as possible so the copy still
doesn't have write permission even though it's now on writable media.

This is the case on at least one of the operating systems
involved. I think this is true of Linux since I haven't used windows
in ages, so I wouldn't be remembering an oddity like this from Windows.

It doesn't sound like you copied the data on your partition off of a CD
(the CD copy you're talking about is something else), but is it
possible that you did something like that with your own data at some
point (e.g. restoring from a backup)?

--Ken

-- 
Ken (Chanoch) Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/
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