linux-ext4 - Re: generic question: user-only directory w/o root access

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Message-ID: <ml2mlm$uqs$1@ger.gmane.org>
Date:	Mon, 8 Jun 2015 02:12:38 +0200
From:	"U.Mutlu" <for-gmane@...luit.com>
To:	linux-ext4@...r.kernel.org
Subject: Re: generic question: user-only directory w/o root access

Theodore Ts'o wrote on 06/06/2015 09:48 PM:
> On Sat, Jun 06, 2015 at 07:46:14PM +0200, U.Mutlu wrote:
>> Theodore Ts'o wrote on 06/06/2015 05:42 PM:
>>> On Sat, Jun 06, 2015 at 09:19:40AM +0200, U.Mutlu wrote:
>>>> I posted hello.c (a FUSE demo) in this thread. It is IMO even more secure
>>>> than the private namespace mount method. The simple reason is:
>>>> because granting access to the volume (or to a single dir/file)
>>>> is done inside that user-code itself, ie. the user/owner controls
>>>> whom he actually gives access.
>>>> I'm sorry to say this, but this simply proves your last statement above wrong.
>>>
>>> So the root user ptraces the FUSE daemon, and it's all she wrote.
>>
>> Protection against tracing and debugging:
>> inside the user-application ie. here the FUSE-client,
>> and also inside the FUSE daemon:
>>
>>    ptrace(PT_DENY_ATTACH, 0, 0, 0);
>>
>> Of course one would need to recompile the FUSE daemon.
>> The company can enforce such a security policy.
>
> And so the root user can install a kernel module which toggles the
> PT_DENY_ATTACH flag for the FUSE daemon after it's started.  Or it
> could use a kprobe or jprobe to dynamically patch the ptrace system
> call to cause it to disregard that flag.  (Or use the ksplice
> funcionality which would make life even easier.)

I could be wrong but I think the DENY_ATTACH is under the control
of the running app itself.
Not sure if any other process can change that on behalf of the app.

>> And while we are at it, I would add a new option to the FUSE daemon,
>> so that the client-app can query it before issuing the mount call,
>> whether it has that protection built in or not, and proceed accordingly...
>> IMO a solvable problem.
>
> And root can cause the kernel to lie to client applications.
>
> Next?

User or his app could check the hash of the kernel file to be sure
it's still the official kernel.



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