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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 18:45:35 +0300

On 26.10.2009 18:26, Pavel Machek wrote:
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 :-).

guest abuses ptrace permissions on his own processes to write to ANY file open by his processes, whose permissions explicitly allow writing to it. Doesn't it trouble you, that guest's processes still retain open file descriptors and hence access to files, that you believe should no longer be accessible to those processes due to permissions you set?
--

Sincerely Your, Dan.


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