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To: Pavel Machek <pavel@ucw.cz>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>, Jan Kara <jack@suse.cz>,
       "J. Bruce Fields" <bfields@fieldses.org>,
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       jamie@shareable.org
Subject: Re: symlinks with permissions
References: <20091025062953.GC1391@ucw.cz>
	<20091026163157.GB7233@duck.suse.cz>
	<20091026165729.GF23564@us.ibm.com>
	<20091026173629.GB16861@fieldses.org>
	<20091026174631.GD7233@duck.suse.cz>
	<1256579869.8576.7.camel@heimdal.trondhjem.org>
	<20091025093604.GA1501@ucw.cz>
From: ebiederm@xmission.com (Eric W. Biederman)
Date: Tue, 27 Oct 2009 21:15:54 -0700
In-Reply-To: <20091025093604.GA1501@ucw.cz> (Pavel Machek's message of "Sun\, 25 Oct 2009 10\:36\:04 +0100")
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Pavel Machek <pavel@ucw.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.

> 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.

Eric
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