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

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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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), then you still have the potential 
for a DoS attack.

Trust me, I've done this before (quite a while back when I was just 
starting out with programming on Linux) with hard-link cycles in an ext4 
filesystem in a virtual machine just to see what would happen (IIRC, 
something deadlocked, I can't remember though if it was fsck or trying 
to access the file once the FS was mounted) (and in fact, I think I may 
try this again just to see if anything has changed).


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