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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 20:19:48 +0400

On 24.10.2009 10:47, Anton Ivanov wrote:
Following your logic we should all abandon directory permissions and
stick to file-only ones. Hmm... Dunno, probably the blood level in my
coffee subsystem is too high this morning, but I do not quite relish
that idea.

I didn't affirm that. I only told, that directory permissions can't in fact restrict access to the file it contains, they can only hamper accessing that file via that directory.

There is a very valid case of trying to restrict access via directory
permissions. Suppose you have a binary program that uses its own
directory but for whatever reason keeps scribbling in files with wrong
permission in it. While I cannot think of a current example, out of the
older ones at least one of the Word Perfect versions for linux used to
do that.

By tightening up the protection on the directory the sysadmin can
mitigate the problem. It is in fact the standard way of doing this.

If the application sets wrong permissions on files, it is by definition broken. Yes, setting more restrictive directory permissions can to some extent mitigate the problem, but not really fix it. What if that application is used by multiple users? The problem raised in the original mail is to some extent artificial, as the only users able to access /proc/<PID>/fd/ are the user with the same UID, as the process EUID, and root, and if the process is either setuid or setgid, /proc/<PID>/fd of that process is accessible only by root. Not to tell about that /proc/<PID>/fd/ contains only symbolic links, not files, so I can't understand, how the original reporter managed to gain access to the file in the restricted directory using that symlink.
--

Sincerely Your, Dan.


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