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Message-ID: <m1zkhh3l83.fsf@fess.ebiederm.org> Date: Mon, 03 Oct 2011 12:53:48 -0700 From: ebiederm@...ssion.com (Eric W. Biederman) To: "Serge E. Hallyn" <serge.hallyn@...onical.com> Cc: Vasiliy Kulikov <segoon@...nwall.com>, Serge Hallyn <serge@...lyn.com>, akpm@...l.org, linux-kernel@...r.kernel.org, netdev@...r.kernel.org, containers@...ts.linux-foundation.org, dhowells@...hat.com, rdunlap@...otime.net, kernel-hardening@...ts.openwall.com Subject: Re: [PATCH 01/15] add Documentation/namespaces/user_namespace.txt (v3) ebiederm@...ssion.com (Eric W. Biederman) writes: > "Serge E. Hallyn" <serge.hallyn@...onical.com> writes: > >> Quoting Vasiliy Kulikov (segoon@...nwall.com): >>> On Tue, Sep 27, 2011 at 08:21 -0500, Serge E. Hallyn wrote: >>> > > First, the patches by design expose much kernel code to unprivileged >>> > > userspace processes. This code doesn't expect malformed data (e.g. VFS, >>> > > specific filesystems, block layer, char drivers, sysadmin part of LSMs, >>> > > etc. etc.). By relaxing permission rules you greatly increase attack >>> > > surface of the kernel from unprivileged users. Are you (or somebody >>> > > else) planning to audit this code? > > Well in theory this codes does expose this code to unprivileged user > space in a way that increases the attack surface. However right now > there are a lot of cases where because the kernel lacks a sufficient > mechanism people are just given root provileges so that can get things > done. Network manager controlling the network stack as an unprivileged > user. Random filesystems on usb sticks being mounted and unmounted > automatically when the usb sticks are inserted and removed. > > I completely agree that auditing and looking at the code is necessary I > think most of what will happen is that we will start directly supporting > how the kernel is actually used in the real world. Which should > actually reduce our level of vulnerability, because we give up the > delusion that large classes of operations don't need careful > attention because only root can perform them. Operations which the > user space authors turn around and write a suid binary for and > unprivileged user space performs those operations all day long. > >>> > I had wanted to (but didn't) propose a discussion at ksummit about how >>> > best to approach the filesystem code. That's not even just for user >>> > namespaces - patches have been floated in the past to make mount an >>> > unprivileged operation depending on the FS and the user's permission >>> > over the device and target. >>> >>> This is a dangerous operation by itself. >> >> Of course it is :) And it's been a while since it has been brought up, >> but it *was* quite well thought through and throrougly discussed - see >> i.e. https://lkml.org/lkml/2008/1/8/131 >> >> Oh, that's right. In the end the reason it didn't go in had to do with >> the ability for an unprivileged user to prevent a privileged user from >> unmounting trees by leaving a busy mount in a hidden namespace. >> >> Eric, in the past we didn't know what to do about that, but I wonder >> if setns could be used in some clever way to solve it from userspace. > > Oh. That is a good objection. I had not realized that unprivileged > mounts had that problem. I just re-read the discussion you are referring to and that wasn't it. Fuse already has something like a revoke in it's umount -f implementation. Eric
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