linux-kernel - Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write

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Date:   Fri, 7 Apr 2017 22:44:39 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     Mathias Krause <minipli@...glemail.com>,
        Kees Cook <keescook@...omium.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Mark Rutland <mark.rutland@....com>,
        Hoeun Ryu <hoeun.ryu@...il.com>,
        PaX Team <pageexec@...email.hu>,
        Emese Revfy <re.emese@...il.com>,
        Russell King <linux@...linux.org.uk>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement
 __arch_rare_write_begin/unmap()

On Fri, 7 Apr 2017, Andy Lutomirski wrote:
> On Fri, Apr 7, 2017 at 6:30 AM, Mathias Krause <minipli@...glemail.com> wrote:
> > On 7 April 2017 at 15:14, Thomas Gleixner <tglx@...utronix.de> wrote:
> >> I really do not care whether PaX wants to chase and verify that over and
> >> over. I certainly don't want to take the chance to leak CR0.WP ever and I
> >> very much care about extra stuff to check in the entry/exit path.
> >
> > 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.

:)

> 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.

As I said before. It should be fused to 1.

> 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.)

That works too, though I'm not sure that it's more efficient than
temporarily creating and undoing a single cpu local alias mapping similar
to the kmap_atomic mechanism in the highmem case.

Aside of that the alias mapping requires a full PGDIR entry unless you want
to get into the mess of keeping yet another identity mapping up to date.

Thanks,

	tglx