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Date: Fri, 7 Apr 2017 21:58:46 -0700
From: Andy Lutomirski <>
To: PaX Team <>
Cc: Mathias Krause <>, Andy Lutomirski <>, 
	Thomas Gleixner <>, Kees Cook <>, 
	"" <>, Mark Rutland <>, 
	Hoeun Ryu <>, Emese Revfy <>, 
	Russell King <>, X86 ML <>, 
	"" <>, 
	"" <>, 
	Peter Zijlstra <>
Subject: Re: Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap()

On Fri, Apr 7, 2017 at 12:58 PM, PaX Team <> wrote:
> On 7 Apr 2017 at 9:14, Andy Lutomirski wrote:
>> On Fri, Apr 7, 2017 at 6:30 AM, Mathias Krause <> wrote:
>> > On 7 April 2017 at 15:14, Thomas Gleixner <> wrote:
>> >> On Fri, 7 Apr 2017, Mathias Krause wrote:
>> > Fair enough. However, placing a BUG_ON(!(read_cr0() & X86_CR0_WP))
>> > somewhere sensible should make those "leaks" visible fast -- and their
>> > exploitation impossible, i.e. fail hard.
>> The leaks surely exist and now we'll just add an exploitable BUG.
> can you please share those leaks that 'surely exist' and CC oss-security
> while at it?

I meant in the patchset here, not in grsecurity.  grsecurity (on very,
very brief inspection) seems to read cr0 and fix it up in

>> Then someone who cares about performance can benchmark the CR0.WP
>> approach against it and try to argue that it's a good idea.  This
>> benchmark should wait until I'm done with my PCID work, because PCID
>> is going to make use_mm() a whole heck of a lot faster.
> in my measurements switching PCID is hovers around 230 cycles for snb-ivb
> and 200-220 for hsw-skl whereas cr0 writes are around 230-240 cycles. there's
> of course a whole lot more impact for switching address spaces so it'll never
> be fast enough to beat cr0.wp.

If I'm reading this right, you're saying that a non-flushing CR3 write
is about the same cost as a CR0.WP write.  If so, then why should CR0
be preferred over the (arch-neutral) CR3 approach?  And why would
switching address spaces obviously be much slower?  There'll be a very
small number of TLB fills needed for the actual protected access.


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