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

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Message-ID: <20200923151835.GA32555@duo.ucw.cz>
Date:   Wed, 23 Sep 2020 17:18:35 +0200
From:   Pavel Machek <pavel@....cz>
To:     Solar Designer <solar@...nwall.com>
Cc:     madvenka@...ux.microsoft.com, 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,
        Rich Felker <dalias@...c.org>
Subject: Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor

Hi!

> > > > 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.
> > 
> > On one hand, Pavel is absolutely right.  It is ridiculous to say that
> > "these are all security holes as an attacker can exploit them to inject
> > his own code."
> 
> I stand corrected, due to Brad's tweet and follow-ups here:
> 
> https://twitter.com/spendergrsec/status/1308728284390318082
> 
> It sure does make sense to combine ret2libc/ROP to mprotect() with one's
> own injected shellcode.  Compared to doing everything from ROP, this is
> easier and more reliable across versions/builds if the desired
> payload

Ok, so this starts to be a bit confusing.

I thought W^X is to protect from attackers that have overflowed buffer
somewhere, but can not to do arbitrary syscalls, yet.

You are saying that there's important class of attackers that can do
some syscalls but not arbitrary ones.

I'd like to see definition of that attacker (and perhaps description
of the system the protection is expected to be useful on -- if it is
not close to common Linux distros).

Best regards,

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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