Re: [PATCH v3 0/7] User namespace mount updates

[Serge Hallyn <serge.hallyn@ubuntu.com>]

Quoting Theodore Ts'o (tytso@mit.edu):
> On Tue, Nov 17, 2015 at 12:34:44PM -0600, Seth Forshee wrote:
> > On Tue, Nov 17, 2015 at 05:55:06PM +0000, Al Viro wrote:
> > > On Tue, Nov 17, 2015 at 11:25:51AM -0600, Seth Forshee wrote:
> > > 
> > > > Shortly after that I plan to follow with support for ext4. I've been
> > > > fuzzing ext4 for a while now and it has held up well, and I'm currently
> > > > working on hand-crafted attacks. Ted has commented privately (to others,
> > > > not to me personally) that he will fix bugs for such attacks, though I
> > > > haven't seen any public comments to that effect.
> > > 
> > > _Static_ attacks, or change-image-under-mounted-fs attacks?
> > 
> > Right now only static attacks, change-image-under-mounted-fs attacks
> > will be next.
> 
> I will fix bugs about static attacks.  That is, it's interesting to me
> that a buggy file system (no matter how it is created), not cause the
> kernel to crash --- and privilege escalation attacks tend to be
> strongly related to those bugs where we're not doing strong enough
> checking.
> 
> Protecting against a malicious user which changes the image under the
> file system is a whole other kettle of fish.  I am not at all user you
> can do this without completely sacrificing performance or making the
> code impossible to maintain.  So my comments do *not* extend to
> protecting against a malicious user who is changing the block device
> underneath the kernel.

Yup, thanks, Ted.  I think the only sane thing to do is work on making the
mounted files immutable.  Guarding against under-mounted-writes seems
crazy.  Well, actually it seems like a fascinating problem, and maybe
solvable without fs changes, but not in scope here.

> If you want to submit patches to make the kernel more robust against
> these attacks, I'm certainly willing to look at the patches.  But I'm
> certainly not guaranteeing that they will go in, and I'm certainly not
> promising to fix all vulnerabilities that you might find that are
> caused by a malicious block device.  Sorry, that's too much buying a
> pig in a poke....
> 
> 						- Ted
>