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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 02:31:47 +0400

On 24.10.2009 1:56, Pavel Machek wrote:
Now... go back to my original email:

%pavel () toy:/tmp/my_priv$ chmod 700 .
%# relax file permissions, directory is private, so this is safe
%# check link count on unwritable_file. We would not want someone
%# to have a hard link to work around our permissions, would we?
%pavel () toy:/tmp/my_priv$ chmod 666 unwritable_file

Yes, you are right, open file descriptor acts as a kind of hardlink
here. Except that

a) this kind of hardlink does not exist when /proc is mounted (and on
non-Linux)

b) unlike other hardlinks, you can't see it on the link count

(and c) writing to file descriptor opened read-only is bad).

Plus, you may run traditional unix/POSIX application, expecting
directory access controls to prevent the write. (Or can you see a way
to write to that file when /proc is unmounted?)

Directory permissions control an access just to the directory
itself, not to the files in it, so your pretensions are in fact
illegitimate.

Demonstrate how to get access to the file with /proc unmounted and you
have a point. Demonstrate how to get access on anything else then
Linux and you have a point. Otherwise there's a security hole.

Did you think of creating a hardlink to the file in an unrestricted location?
That is the like "security hole".
--

Sincerely Your, Dan.


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