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Date: Sun, 28 Oct 2018 19:31:26 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Kees Cook <keescook@...omium.org>
Cc: Igor Stoppa <igor.stoppa@...il.com>,
Mimi Zohar <zohar@...ux.vnet.ibm.com>,
Matthew Wilcox <willy@...radead.org>,
Dave Chinner <david@...morbit.com>,
James Morris <jmorris@...ei.org>, Michal Hocko <mhocko@...nel.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
linux-integrity <linux-integrity@...r.kernel.org>,
linux-security-module <linux-security-module@...r.kernel.org>,
Igor Stoppa <igor.stoppa@...wei.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Jonathan Corbet <corbet@....net>, Laura Abbott <labbott@...hat.com>,
Randy Dunlap <rdunlap@...radead.org>,
Mike Rapoport <rppt@...ux.vnet.ibm.com>,
"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/17] prmem: documentation
On Fri, Oct 26, 2018 at 11:46:28AM +0100, Kees Cook wrote:
> On Fri, Oct 26, 2018 at 10:26 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > I still don't really understand the whole write-rare thing; how does it
> > really help? If we can write in kernel memory, we can write to
> > page-tables too.
> One aspect of hardening the kernel against attack is reducing the
> internal attack surface. Not all flaws are created equal, so there is
> variation in what limitations an attacker may have when exploiting
> flaws (not many flaws end up being a fully controlled "write anything,
> anywhere, at any time"). By making the more sensitive data structures
> of the kernel read-only, we reduce the risk of an attacker finding a
> path to manipulating the kernel's behavior in a significant way.
>
> Examples of typical sensitive targets are function pointers, security
> policy, and page tables. Having these "read only at rest" makes them
> much harder to control by an attacker using memory integrity flaws.
Because 'write-anywhere' exploits are easier than (and the typical first
step to) arbitrary code execution thingies?
> The "write rarely" name itself may not sufficiently describe what is
> wanted either (I'll take the blame for the inaccurate name), so I'm
> open to new ideas there. The implementation requirements for the
> "sensitive data read-only at rest" feature are rather tricky:
>
> - allow writes only from specific places in the kernel
> - keep those locations inline to avoid making them trivial ROP targets
> - keep the writeability window open only to a single uninterruptable CPU
The current patch set does not achieve that because it uses a global
address space for the alias mapping (vmap) which is equally accessible
from all CPUs.
> - fast enough to deal with page table updates
The proposed implementation needs page-tables for the alias; I don't see
how you could ever do R/O page-tables when you need page-tables to
modify your page-tables.
And this is entirely irrespective of performance.
> The proposal I made a while back only covered .data things (and used
> x86-specific features).
Oh, right, that CR0.WP stuff.
> Igor's proposal builds on this by including a
> way to do this with dynamic allocation too, which greatly expands the
> scope of structures that can be protected. Given that the x86-only
> method of write-window creation was firmly rejected, this is a new
> proposal for how to do it (vmap window). Using switch_mm() has also
> been suggested, etc.
Right... /me goes find the patches we did for text_poke. Hmm, those
never seem to have made it:
https://lkml.kernel.org/r/20180902173224.30606-1-namit@vmware.com
like that. That approach will in fact work and not be a completely
broken mess like this thing.
> We need to find a good way to do the write-windowing that works well
> for static and dynamic structures _and_ for the page tables... this
> continues to be tricky.
>
> Making it resilient against ROP-style targets makes it difficult to
> deal with certain data structures (like list manipulation). In my
> earlier RFC, I tried to provide enough examples of where this could
> get used to let people see some of the complexity[1]. Igor's series
> expands this to even more examples using dynamic allocation.
Doing 2 CR3 writes for 'every' WR write doesn't seem like it would be
fast enough for much of anything.
And I don't suppose we can take the WP fault and then fix up from there,
because if we're doing R/O page-tables, that'll incrase the fault depth
and we'll double fault all the time, and tripple fault where we
currently double fault. And we all know how _awesome_ tripple faults
are.
But duplicating (and wrapping in gunk) whole APIs is just not going to
work.
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