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Re: /proc filesystem allows bypassing directory permissions on Linux
From: Dan Yefimov <dan () lightwave net ru>
Date: Sat, 24 Oct 2009 02:55:12 +0400

On 24.10.2009 2:39, Pavel Machek wrote:
Original owner did chmod 666... after making sure traditional unix
permissions protect the file. Please look at original mail; it was
subtle but I believe I got it right, and file would not be writable
with /proc unmounted.

I remember the original mail content. You're right, you can't reach
the file if the procfs is not mounted, but you forget about the
race, allowing the guest to create a hardlink to the file in an
unrestricted location before the directory access becomes
restricted. Again, procfs is just another, specific kind of
hardlinks.

Check it again.  There's no race; I check link count before chmod 666.

I didn't see real commands checking the link count, just comments telling about it. Not to tell about your script is broken by design. With what object do you 'chmod 0666 unwritable_file', if that file is not designed for access by anybody other than you? This is a rhetorical question.
--

Sincerely Your, Dan.


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