Re: [PATCH 01/15] add Documentation/namespaces/user_namespace.txt (v3)

From: Eric W. Biederman
Date: Mon Oct 03 2011 - 15:53:53 EST

ebiederm@xxxxxxxxxxxx (Eric W. Biederman) writes:

> "Serge E. Hallyn" <serge.hallyn@xxxxxxxxxxxxx> writes:
>> Quoting Vasiliy Kulikov (segoon@xxxxxxxxxxxx):
>>> 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.
>> 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

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