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Re: /proc filesystem allows bypassing directory permissions on Linux
From: Pavel Machek <pavel () ucw cz>
Date: Mon, 26 Oct 2009 16:26:09 +0100

On Mon 2009-10-26 18:11:56, Dan Yefimov wrote:
On 26.10.2009 18:06, Pavel Machek wrote:
On Mon 2009-10-26 15:37:50, Dan Yefimov wrote:
On 26.10.2009 13:54, psz () maths usyd edu au wrote:
Dear Dan,

... in authentic kernels /proc/<PID>/fd/<FD>   are symlinks ...

They appear to /bin/ls as symlinks, but observation suggests that they
"act" as hardlinks. Could that be fixed somehow? (I did look at the
kernel fs/proc/base.c but did not make much sense to me...)

Just looked more carefully at fs/proc/base.c. That behavior is due
to proc_fd_info() called from proc_fd_link() obtains file->f_path,
that in turn contains the reference to the open file dentry and
hence inode. That's exactly why those symlinks behave as hardlinks.
This behavior assumes, that if you were able to open the file,
you've all necessary transition permissions to access it's inode.
But in order to follow them you need privileges to read the process
memory, which hardly restricts the impact of this behavior. I don't
think this should be fixed, since /proc/<PID>/fd/ is mainly for
debugging purposes.

guest certianly does not have permission to ptrace() pavel's
processes, so...

But guest has permissions to ptrace() his own processes. If we
remember your original report, he abuses input redirection of bash
run by himself. So again, there's no real security hole here.

guest abuses ptrace permissions on his own processes to write to
pavel's files... no, that obviously is not security hole :-).

Whatever. I agree that it is obscure, but I believe that it is
security problem, and the one that should be fixed. By now, someone is
probably working for a fix. (I took a stab at patching kernel, too).

We probably should not continue this debate on bugtraq. Enough was
said already, and right people are probably already aware of this
issue.

(The debate is still relevant on lkml, so lets continue discussion
there).
                                                                        Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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