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Date:   Wed, 23 Sep 2020 16:11:02 +0200
From:   Solar Designer <solar@openwall.com>
To:     Pavel Machek <pavel@ucw.cz>
Cc:     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, mic@digikod.net,
        Rich Felker <dalias@libc.org>
Subject: Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor
Message-ID: <20200923141102.GA7142@openwall.com>
References: <20200922215326.4603-1-madvenka@linux.microsoft.com> <20200923081426.GA30279@amd> <20200923091456.GA6177@openwall.com>
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On Wed, Sep 23, 2020 at 11:14:56AM +0200, Solar Designer wrote:
> On Wed, Sep 23, 2020 at 10:14:26AM +0200, Pavel Machek wrote:
> > > Introduction
> > > ============
> > > 
> > > Dynamic code is used in many different user applications. Dynamic code is
> > > often generated at runtime. Dynamic code can also just be a pre-defined
> > > sequence of machine instructions in a data buffer. Examples of dynamic
> > > code are trampolines, JIT code, DBT code, etc.
> > > 
> > > Dynamic code is placed either in a data page or in a stack page. In order
> > > to execute dynamic code, the page it resides in needs to be mapped with
> > > execute permissions. Writable pages with execute permissions provide an
> > > attack surface for hackers. Attackers can use this to inject malicious
> > > code, modify existing code or do other harm.
> > > 
> > > To mitigate this, LSMs such as SELinux implement W^X. That is, they may not
> > > allow pages to have both write and execute permissions. This prevents
> > > dynamic code from executing and blocks applications that use it. To allow
> > > genuine applications to run, exceptions have to be made for them (by setting
> > > execmem, etc) which opens the door to security issues.
> > > 
> > > 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
is non-trivial.  My own example: invoking a shell in a local attack on
Linux is trivial enough to do via ret2libc only, but a connect-back
shell in a remote attack might be easier and more reliably done via
mprotect() + shellcode.

Per the follow-ups, this was an established technique on Windows and iOS
until further hardening prevented it.  So it does make sense for Linux
to do the same (as an option because of it breaking existing stuff), and
not so much as policy enforcement for the sake of it and ease of
reasoning, but mostly to force real-world exploits to be more complex
and less reliable.

> On the other hand, "what W^X is supposed to protect from" depends on how
> the term W^X is defined (historically, by PaX and OpenBSD).  It may be
> that W^X is partially not a feature to defeat attacks per se, but also a
> policy enforcement feature preventing use of dangerous techniques (JIT).
> 
> Such policy might or might not make sense.  It might make sense for ease
> of reasoning, e.g. "I've flipped this setting, and now I'm certain the
> system doesn't have JIT within a process (can still have it through
> dynamically creating and invoking an entire new program), so there are
> no opportunities for an attacker to inject code nor generate previously
> non-existing ROP gadgets into an executable mapping within a process."
> 
> I do find it questionable whether such policy and such reasoning make
> sense beyond academia.

I was wrong in the above, focusing on the wrong thing.

> Then, there might be even more ways in which W^X is not perfect enough
> to enable such reasoning.  What about using ptrace(2) to inject code?
> Should enabling W^X also disable ability to debug programs by non-root?
> We already have Yama ptrace_scope, which can achieve that at the highest
> setting, although that's rather inconvenient and is probably unexpected
> by most to be a requirement for having (ridiculously?) full W^X allowing
> for the academic reasoning.

Thinking out loud:

Technically, ptrace() is also usable from a ROP chain.  It might be too
cumbersome to bother using to get a shellcode going, but OTOH it's just
one function to be invoked in a similar fashion multiple times, so might
be more reliable than having a ROP chain depend on multiple actually
needed functions directly (moving that dependency into the shellcode).

> Personally, I am for policies that make more practical sense.  For
> example, years ago I advocated here on kernel-hardening that we should
> have a mode where ELF flags enabling/disabling executable stack are
> ignored, and non-executable stack is always enforced.  This should also
> be extended to default (at program startup) permissions on more than
> just stack (but also on .bss, typical libcs' heap allocations, etc.)
> However, I am not convinced there's enough value in extending the policy
> to restricting explicit uses of mprotect(2).
> 
> Yes, PaX did that, and its emutramp.txt said "runtime code generation is
> by its nature incompatible with PaX's PAGEEXEC/SEGMEXEC and MPROTECT
> features, therefore the real solution is not in emulation but by
> designing a kernel API for runtime code generation and modifying
> userland to make use of it."  However, not being convinced in the
> MPROTECT feature having enough practical value,

I am convinced now, however:

> I am also not convinced
> "a kernel API for runtime code generation and modifying userland to make
> use of it" is the way to go.

doesn't automatically follow from the above, because:

> Having static instead of dynamically-generated trampolines in userland
> code where possible (and making other userland/ABI changes to make that
> possible in more/all cases) is an obvious improvement, and IMO should be
> a priority over the above.
> 
> While I share my opinion here, I don't mean that to block Madhavan's
> work.  I'd rather defer to people more knowledgeable in current userland
> and ABI issues/limitations and plans on dealing with those, especially
> to Florian Weimer.  I haven't seen Florian say anything specific for or
> against Madhavan's proposal, and I'd like to.  (Have I missed that?)
> It'd be wrong to introduce a kernel API that userland doesn't need, and
> it'd be right to introduce one that userland actually intends to use.
> 
> I've also added Rich Felker to CC here, for musl libc and its possible
> intent to use the proposed API.  (My guess is there's no such need, and
> thus no intent, but Rich might want to confirm that or correct me.)

So need to hear more from the userland folks, I guess.

Alexander