linux-kernel - Re: symlinks with permissions

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Date:	Mon, 26 Oct 2009 11:57:29 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Jan Kara <jack@...e.cz>
Cc:	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

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.

-serge
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