Messages in this thread | | | Subject | Re: [PATCH 0/7] Initial support for user namespace owned mounts | From | Austin S Hemmelgarn <> | Date | Wed, 22 Jul 2015 12:52:58 -0400 |
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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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