Re: [PATCH 01/15] add Documentation/namespaces/user_namespace.txt(v3)
From: Serge E. Hallyn
Date: Mon Oct 03 2011 - 16:04:53 EST
Quoting Eric W. Biederman (ebiederm@xxxxxxxxxxxx):
> 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. 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
The one I linked was one discussion, but not the final one.
https://lkml.org/lkml/2008/10/6/72 is the one where the need for
revoke is brought up.
> it. Fuse already has something like a revoke in it's umount -f
I'll have to (haven't yet) take a look at it.
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