linux-kernel - Re: symlinks with permissions

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Date:	Wed, 28 Oct 2009 04:25:23 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Pavel Machek <pavel@....cz>
Cc:	Trond Myklebust <trond.myklebust@....uio.no>,
	Jan Kara <jack@...e.cz>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	kernel list <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	jamie@...reable.org
Subject: Re: symlinks with permissions

Pavel Machek <pavel@....cz> writes:

> On Tue 2009-10-27 21:15:54, Eric W. Biederman wrote:
>> Pavel Machek <pavel@....cz> writes:
>> 
>> > On Mon 2009-10-26 13:57:49, Trond Myklebust wrote:
>> >> On Mon, 2009-10-26 at 18:46 +0100, Jan Kara wrote:
>> >> >   That's what I'd think as well but it does not as I've just learned and
>> >> > tested :) proc_pid_follow_link actually directly gives a dentry of the
>> >> > target file without checking permissions on the way.
>> >
>> > It is weider. That symlink even has permissions. Those are not
>> > checked, either.
>> >  
>> >> I seem to remember that is deliberate, the point being that a symlink
>> >> in /proc/*/fd/ may contain a path that refers to a private namespace.
>> >
>> > Well, it is unexpected and mild security hole.
>> 
>> /proc/<pid>/fd is only viewable by the owner of the process or by
>> someone with CAP_DAC_OVERRIDE.  So there appears to be no security
>> hole exploitable by people who don't have the file open.
>
> Please see bugtraq discussion at
> http://seclists.org/bugtraq/2009/Oct/179 .
>
> (In short, you get read-only fd, and you can upgrade it to read-write
> fd. Yes, you are the owner of the process, but you are not owner of
> the file the fd refers to.)

Assuming you have permission to open it read-write.

>> > Part of the problem is that even  if you have read-only
>> > filedescriptor, you can upgrade it to read-write, even if path is
>> > inaccessible to you.
>> >
>> > So if someone passes you read-only filedescriptor, you can still write
>> > to it.
>> 
>> Openly if you actually have permission to open the file again.  The actual
>> permissions on the file should not be ignored.
>
> The actual permissions of the file are not ignored, but permissions of
> the containing directory _are_. If there's 666 file in 700 directory,
> you can reopen it read-write, in violation of directory's 700
> permissions.

I can see how all of this can come as a surprise.  However I don't see
how any coder who is taking security seriously and being paranoid about
security would actually write code that would have a problem with this.

Do you know of any cases where this difference matters in practice?

It looks to me like it has been this way for better than a decade
without problems so there is no point in changing it now.  

Eric
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