Re: [PATCH 0/7] Initial support for user namespace owned mounts

J. Bruce Fields
2015-07-23T13:19:38Z

On Thu, Jul 23, 2015 at 11:51:35AM +1000, Dave Chinner wrote:
> On Wed, Jul 22, 2015 at 01:41:00PM -0400, J. Bruce Fields wrote:
> > On Wed, Jul 22, 2015 at 12:52:58PM -0400, Austin S Hemmelgarn wrote:
> > > On 2015-07-22 10:09, J. Bruce Fields wrote:
> > > >On Wed, Jul 22, 2015 at 05:56:40PM +1000, Dave Chinner wrote:
> > > >>On Tue, Jul 21, 2015 at 01:37:21PM -0400, J. Bruce Fields wrote:
> > > >>>On Fri, Jul 17, 2015 at 12:47:35PM +1000, Dave Chinner wrote:
> > > >>>So, for example, a screwed up on-disk directory structure shouldn't
> > > >>>result in creating a cycle in the dcache and then deadlocking.
> > > >>
> > > >>Therein lies the problem: how do you detect such structural defects
> > > >>without doing a full structure validation?
> > > >
> > > >You can prevent cycles in a graph if you can prevent adding an edge
> > > >which would be part of a cycle.
> > > >
> > > Except if the user can write to the filesystem's backing storage (be
> > > it a device or a file), and has sufficient knowledge of the on-disk
> > > structures, they can create all the cycles they want in the
> > > metadata. So unless the kernel builds the graph internally by
> > > parsing the metadata _and_ has some way to detect that the on-disk
> > > metadata has hit a cycle (which may not just involve 2 items),
> >
> > Understood. Again, see the d_ancestor call in d_splice_alias, this is
> > exactly what it checks for.
>
> But that only addresses one type of loop in one specific metadata
> structure.

Yep, agreed!

> There's plenty of other ways you could construct metadata
> loops that are essentially undetected and result in either deadlock
> or livelock within the filesystem code itself. e.g. just make btree
> sibling pointers loop over a range of entries that have the same
> index key (e.g. free space extents of the same size). If allocation
> then falls into this loop, the kernel will just spin searching the
> same blocks for something it will never find. Such resource
> consumption attacks are trivial to construct but extremely difficult
> to detect because they exploit normal behaviour of the structure and
> algorithms by mangling trusted pointers.

Interesting example, thanks! I doubt this particular example would be
*that* hard to detect? But understood that there may be lots of others.

--b.

>
> Of course, this sort of attack will eventually deadlock the
> filesystem because it will backs up on locks held by the live locked
> search. Once the filesystem is deadlocked, it can then cause sync()
> calls to get stuck on the filesystem. And because sync() is a global
> operation, a deadlocked filesystem in one container could cause sync
> to hang in completely unrelated container....
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com