linux-kernel - Re: symlinks with permissions

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20091026173629.GB16861@fieldses.org>
Date:	Mon, 26 Oct 2009 13:36:29 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	Jan Kara <jack@...e.cz>, 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 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....

--b.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/