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

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Date:	Wed, 21 Oct 2015 13:49:07 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	Borislav Petkov <bp@...en8.de>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <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" <linux-efi@...r.kernel.org>
Subject: Re: [PATCH v2] x86/mm: warn on W+x mappings

On Wed, Oct 21, 2015 at 1:45 PM, Matt Fleming <matt@...eblueprint.co.uk> wrote:
> On Wed, 21 Oct, at 11:46:53AM, Andy Lutomirski wrote:
>>
>> If the UEFI stuff is mapped in its own PGD entry, we could just RO
>> that entire PGD entry everywhere except the UEFI pgd (and make sure to
>> clear G so that the TLB entries get zapped).
>
> What would be the benefit of making it RO as opposed to not having it
> mapped at all?

Nothing.

> The mappings only exist in the trampoline_pgd right now
> for x86 which minimizes the potentially vulnerable code paths to the
> EFI runtime calls and the suspend/resume code.

Oh, I didn't realize it.

So what's the problem here?  Honestly, while UEFI is full of
questionable things, I don't really see how an unprivileged user
program should be able to cause malicious input to be send to UEFI
code, so it should be quite difficult to exploit a buffer overflow or
other errant write in UEFI to escalate privileges from user to
anything else.  (Kernel -> SMM escalation is a whole different story,
but preventing that is SMM's business, not the kernel's.  I've
actually been a wee bit tempted to write a /dev/smram driver to expose
SMRAM using a portfolio of old known exploits.)

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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