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Date: Fri, 07 Apr 2017 21:58:40 +0200
From: "PaX Team" <>
To: Mathias Krause <>, Andy Lutomirski <>
CC: Thomas Gleixner <>, Kees Cook <>,
        Andy Lutomirski <>,
        "" <>,
        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 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 think we're approaching this all wrong, actually.  The fact that x86
> has this CR0.WP thing is arguably a historical accident, and the fact
> that PaX uses it doesn't mean that PaX is doing it the best way for
> upstream Linux.
> Why don't we start at the other end and do a generic non-arch-specific
> implementation: set up an mm_struct that contains an RW alias of the
> relevant parts of rodata and use use_mm to access it.  (That is,
> get_fs() to back up the old fs, set_fs(USER_DS),
> use_mm(&rare_write_mm), do the write using copy_to_user, undo
> everything.)
> 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.

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