From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=47410 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1Py4Bw-00019o-7k for qemu-devel@nongnu.org; Fri, 11 Mar 2011 10:23:49 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Py4Bu-0001kC-AI for qemu-devel@nongnu.org; Fri, 11 Mar 2011 10:23:47 -0500 Received: from e39.co.us.ibm.com ([32.97.110.160]:40198) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Py4Bu-0001iS-3d for qemu-devel@nongnu.org; Fri, 11 Mar 2011 10:23:46 -0500 Received: from d03relay03.boulder.ibm.com (d03relay03.boulder.ibm.com [9.17.195.228]) by e39.co.us.ibm.com (8.14.4/8.13.1) with ESMTP id p2BFAgEj013638 for ; Fri, 11 Mar 2011 08:10:42 -0700 Received: from d03av02.boulder.ibm.com (d03av02.boulder.ibm.com [9.17.195.168]) by d03relay03.boulder.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id p2BFNVkr114482 for ; Fri, 11 Mar 2011 08:23:34 -0700 Received: from d03av02.boulder.ibm.com (loopback [127.0.0.1]) by d03av02.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id p2BFNURx008484 for ; Fri, 11 Mar 2011 08:23:30 -0700 Message-ID: <4D7A3E6B.6070106@linux.vnet.ibm.com> Date: Fri, 11 Mar 2011 07:23:23 -0800 From: "Venkateswararao Jujjuri (JV)" MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [V8 PATCH 11/11] virtio-9p: Chroot environment for other functions References: <1299690960-12007-1-git-send-email-mohan@in.ibm.com> <1299690960-12007-12-git-send-email-mohan@in.ibm.com> <4D79B90A.2020204@linux.vnet.ibm.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: "M. Mohan Kumar" , qemu-devel@nongnu.org On 3/10/2011 10:30 PM, Stefan Hajnoczi wrote: > On Fri, Mar 11, 2011 at 5:54 AM, Venkateswararao Jujjuri (JV) > wrote: >> On 3/10/2011 4:29 AM, Stefan Hajnoczi wrote: >>> On Wed, Mar 9, 2011 at 5:16 PM, M. Mohan Kumar wrote: >>>> Add chroot functionality for systemcalls that can operate on a file >>>> using relative directory file descriptor. >>> >>> I suspect the relative directory approach is broken and escapes the >>> chroot. Here's why: >>> >>> The request is local_chmod(fs_ctx, "/..", credp). dirname("/..") is >>> "/" and basename("..") is "..". >> >> We should never receive protocol operations with relative path. >> Client should always resolve to full path and send the request. >> If the client is malicious this scenario can be be possible.. but in that case >> it is fine to fail the operation. > > What I haven't audited yet is whether symlinks can be abused in any of > these *at(2) operations. Reading symlink sends only contents to the client. So a symlink can contain anything. But when the fully populated path comes we avoid the potential symlink issue by opening the entire dir in chrooted process. > > The *at(2) approach seems like a shortcut to avoid implementing > individual chroot protocol requests/responses for stat(2) and friends. > But it carries the risk that if we don't use NOFOLLOW then we can be > tricked into escaping the "chroot" because we're performing the > operation outside the chroot. I would agree with your observation. With *at(2) we need the following: 1. The path should never have ".." May be we can check it early enough and fail. 2. Get the pfd from the chroot_thread 3. use NO_FOLLOW. If we do all three then we are fool prof. > > I'll take a look later today to make sure all operations safe traverse > paths outside the chroot. If we move all the operations to chroot, we are essentially serializing all operations. But the code looks lot cleaner though. - JV > > Stefan >