linux-kernel - Re: symlinks with permissions

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Message-ID: <20091026180224.GD16861@fieldses.org>
Date:	Mon, 26 Oct 2009 14:02:24 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Jan Kara <jack@...e.cz>
Cc:	"Serge E. Hallyn" <serue@...ibm.com>, Pavel Machek <pavel@....cz>,
	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

On Mon, Oct 26, 2009 at 06:46:31PM +0100, Jan Kara wrote:
> On Mon 26-10-09 13:36:29, J. Bruce Fields wrote:
> > On Mon, Oct 26, 2009 at 11:57:29AM -0500, Serge E. Hallyn wrote:
> > > Quoting Jan Kara (jack@...e.cz):
> > > >   Hi,
> > > > 
> > > > On Sun 25-10-09 07:29:53, Pavel Machek wrote:
> > > > > ...yes, they do exist, in /proc/self/fd/* . Unfortunately, their
> > > > > permissions are not actually checked during open, resulting in
> > > > > (obscure) security hole: if you have fd open for reading, you can
> > > > > reopen it for write, even through unix permissions would not allow
> > > > > that.
> > > > > 
> > > > > Now... I'd like to close the hole. One way would be to actually check
> > > > > symlink permissions on open -- because those symlinks already have
> > > > > correct permissions.
> > > >   Hmm, I'm not sure I understand the problem. Symlink is just a file
> > > > containing a path. So if you try to open a symlink, you will actually open
> > > > a file to which the path points. So what security problem is here? Either
> > > > you can open the file symlink points to for writing or you cannot...
> > > >   Anyway, if you want to play with this,
> > > > fs/proc/base.c:proc_pid_follow_link
> > > >   is probably the function you are interested in.
> > > 
> > > The problem he's trying to address is that users may try to protect
> > > a file by doing chmod 700 on the parent dir, but leave the file itself
> > > accessible.  They don't realize that merely having a task with an open
> > > fd to that file gives other users another path to the file.
> > > 
> > > Whether or not that's actually a problem is open to debate, but I think
> > > he's right that many users aren't aware of it.
> > 
> > If /proc/self/fd/23 is a symlink to /home/me/privatedir/secret, then an
> > open("proc/self/fd/23",...) still traverses the whole /home/.../secret
> > path, and needs appropriate permissions at each step, doesn't it?
> > 
> > Probably I'm just terminally confused....
>   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.

Got it, thanks.--b.
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