linux-kernel - Re: [PATCH v2] x86/mm: warn on W+x mappings

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Date:	Wed, 21 Oct 2015 14:49:24 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	Borislav Petkov <bp@...en8.de>,
	Stephen Smalley <sds@...ho.nsa.gov>, x86@...nel.org,
	linux-kernel@...r.kernel.org, keescook@...omium.org,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andy Lutomirski <luto@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>, linux-efi@...r.kernel.org,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH v2] x86/mm: warn on W+x mappings


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Matt Fleming <matt@...eblueprint.co.uk> wrote:
> 
> > > > Right, we could do that, but then we wouldn't be able to support 
> > > > creation/updating variables at runtime, such as when you install a 
> > > > distribution for the first time, or want to boot a new kernel filename 
> > > > directly from the firmware without a boot loader (and need to modify the 
> > > > BootXXXX variables).
> > > 
> > > Do we know the precise position and address range of these variables?
> > > 
> > > We could map them writable (but not executable), and the rest executable (but 
> > > not writable).
> >  
> > The variables are stored in NVRAM, which we don't map into the kernel virtual 
> > address space. [...]
> 
> Just curious: is there firmware that memory maps those variables privately?
> 
> > [...] We have to initiate the transaction of writing to the variables by 
> > executing EFI runtime services.
> > 
> > We obviously have buffers that we pass to the BIOS that contain variable data, 
> > but these should be NX anyway because they're regular kernel allocations.
> > 
> > > That raises the question whether the same physical page ever mixes variables 
> > > and actual code - but the hope would be that it's suffiently page granular for 
> > > this to work.
> > 
> > I don't think that would ever happen.
> 
> Ok, that's promising, so how about this then to solve the security weakness the 
> new warning unearthed: map the whole EFI range as 'r-x (NX)', but detect writes 
> from the page fault handler and transparently allow them to flip over the range 
> to 'rw-'.

So I meant to say 'page' instead of 'range'.

I.e. this dynamic mechanism would flip pages over to 'rw-', as write faults occur 
from EFI code that writes to them.

We don't need to know which regions are writable data, and which regions are 
executable-code/readonly-data.

The following aspect would guarantee safety:

> Note that for security reasons we don't allow a subsequent flipping back to NX 
> if there's an NX fault on the same page, i.e. this new mechanism is a monotonic 
> one-way process that should dynamically 'map out' data pages versus executable 
> pages.
> 
> It should also be pretty robust, assuming we can take page faults while EFI code 
> is executing and is trying to modify EFI data: is that the case?

and this is why I asked whether boundaries between 'Code' and 'Writable data' 
sections are page granular - which they do appear to be. (i.e. there are no 
singular pages that are both writable data and code at once.)

Thanks,

	Ingo
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