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Re: /proc filesystem allows bypassing directory permissions on Linux
From: Dan Yefimov <dan () lightwave net ru>
Date: Mon, 26 Oct 2009 15:37:50 +0300

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.
--

Sincerely Your, Dan.


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