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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 22 Jul 2015 12:52:58 -0400
From:	Austin S Hemmelgarn <ahferroin7@...il.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>,
	Dave Chinner <david@...morbit.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Casey Schaufler <casey@...aufler-ca.com>,
	Andy Lutomirski <luto@...capital.net>,
	Seth Forshee <seth.forshee@...onical.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	LSM List <linux-security-module@...r.kernel.org>,
	SELinux-NSA <selinux@...ho.nsa.gov>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/7] Initial support for user namespace owned mounts

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).


Download attachment "smime.p7s" of type "application/pkcs7-signature" (3019 bytes)