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

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Date:   Wed, 23 Sep 2020 13:32:38 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Pavel Machek <pavel@....cz>
Cc:     kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, oleg@...hat.com,
        x86@...nel.org, luto@...nel.org, David.Laight@...LAB.COM,
        fweimer@...hat.com, mark.rutland@....com, mic@...ikod.net
Subject: Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor

...
>> The W^X implementation today is not complete. There exist many user level
>> tricks that can be used to load and execute dynamic code. E.g.,
>>
>> - Load the code into a file and map the file with R-X.
>>
>> - Load the code in an RW- page. Change the permissions to R--. Then,
>>   change the permissions to R-X.
>>
>> - Load the code in an RW- page. Remap the page with R-X to get a separate
>>   mapping to the same underlying physical page.
>>
>> IMO, these are all security holes as an attacker can exploit them to inject
>> his own code.
> 
> IMO, you are smoking crack^H^H very seriously misunderstanding what
> W^X is supposed to protect from.
> 
> W^X is not supposed to protect you from attackers that can already do
> system calls. So loading code into a file then mapping the file as R-X
> is in no way security hole in W^X.
> 
> If you want to provide protection from attackers that _can_ do system
> calls, fine, but please don't talk about W^X and please specify what
> types of attacks you want to prevent and why that's good thing.
> 


There are two things here - the idea behind W^X and the current realization
of that idea in actual implementation. The idea behind W^X, as I understand,
is to prevent a user from loading arbitrary code into a page and getting it
to execute. If the user code contains a vulnerability, an attacker can 
exploit it to potentially inject his own code and get it to execute. This
cannot be denied.

>From that perspective, all of the above tricks I have mentioned are tricks
that user code can use to load arbitrary code into a page and get it to
execute.

Now, I don't want the discussion to be stuck in a mere name. If what I am
suggesting needs a name other than "W^X" in the opinion of the reviewers,
that is fine with me. But I don't believe there is any disagreement that
the above user tricks are security holes.

Madhavan