Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor

From: "Mickaël Salaün" <mic@digikod.net>
To: Pavel Machek <pavel@ucw.cz>
Cc: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org, oleg@redhat.com,
	x86@kernel.org, luto@kernel.org, David.Laight@ACULAB.COM,
	fweimer@redhat.com, mark.rutland@arm.com
Subject: Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor
Date: Fri, 25 Sep 2020 12:12:01 +0200	[thread overview]
Message-ID: <eec5d097-c70f-365b-3548-66726ad49f04@digikod.net> (raw)
In-Reply-To: <20200924220540.GA13185@amd>

On 25/09/2020 00:05, Pavel Machek wrote:
> Hi!
> 
>>>>> I believe you should simply delete confusing "introduction" and
>>>>> provide details of super-secure system where your patches would be
>>>>> useful, instead.
>>>>
>>>> This RFC talks about converting dynamic code (which cannot be authenticated)
>>>> to static code that can be authenticated using signature verification. That
>>>> is the scope of this RFC.
>>>>
>>>> If I have not been clear before, by dynamic code, I mean machine code that is
>>>> dynamic in nature. Scripts are beyond the scope of this RFC.
>>>>
>>>> Also, malware compiled from sources is not dynamic code. That is orthogonal
>>>> to this RFC. If such malware has a valid signature that the kernel permits its
>>>> execution, we have a systemic problem.
>>>>
>>>> I am not saying that script authentication or compiled malware are not problems.
>>>> I am just saying that this RFC is not trying to solve all of the security problems.
>>>> It is trying to define one way to convert dynamic code to static code to address
>>>> one class of problems.
>>>
>>> Well, you don't have to solve all problems at once.
>>>
>>> But solutions have to exist, and AFAIK in this case they don't. You
>>> are armoring doors, but ignoring open windows.
>>
>> FYI, script execution is being addressed (for the kernel part) by this
>> patch series:
>> https://lore.kernel.org/lkml/20200924153228.387737-1-mic@digikod.net/
> 
> Ok.
> 
>>> Or very probably you are thinking about something different than
>>> normal desktop distros (Debian 10). Because on my systems, I have
>>> python, gdb and gcc...
>>
>> It doesn't make sense for a tailored security system to leave all these
>> tools available to an attacker.
> 
> And it also does not make sense to use "trampoline file descriptor" on
> generic system... while W^X should make sense there.

Well, as said before, (full/original/system-wide) W^X may require
trampfd (as well as other building-blocks).

I guess most Linux deployments are not on "generic systems"
anyway (even if they may be based on generic distros), and W^X
contradicts the fact that users/attackers can do whatever they want on
the system.

> 
>>> It would be nice to specify what other pieces need to be present for
>>> this to make sense -- because it makes no sense on Debian 10.
>>
>> Not all kernel features make sense for a generic/undefined usage,
>> especially specific security mechanisms (e.g. SELinux, Smack, Tomoyo,
>> SafeSetID, LoadPin, IMA, IPE, secure/trusted boot, lockdown, etc.), but
>> they can still be definitely useful.
> 
> Yep... so... I'd expect something like... "so you have single-purpose
> system

No one talked about a single-purpose system.

> with all script interpreters removed,

Not necessarily with the patch series I pointed out just before.

> IMA hashing all the files
> to make sure they are not modified, and W^X enabled.

System-wide W^X is not only for memory, and as Madhavan said: "this RFC
pertains to converting dynamic [writable] machine code to static
[non-writable] code".

> Attacker can
> still execute code after buffer overflow by .... and trapoline file
> descriptor addrsses that"... so that people running generic systems
> can stop reading after first sentence.

Are you proposing to add a
"[feature-not-useful-without-a-proper-system-configuration]" tag in
subjects? :)