linux-kernel - Re: symlinks with permissions

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Date:	Tue, 27 Oct 2009 21:15:54 -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 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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