linux-kernel - Re: [PATCH] x86: make IDT read-only

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Message-ID: <CAG7+5M06h_UEy39OQqdT7Z6r-85q_R9zHHKGO6F2PX2pgT6U9g@mail.gmail.com>
Date:	Wed, 10 Apr 2013 09:31:41 -0700
From:	Eric Northup <digitaleric@...gle.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Kees Cook <keescook@...omium.org>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Alex Shi <alex.shi@...el.com>,
	Borislav Petkov <borislav.petkov@....com>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	lf-virt <virtualization@...ts.linux-foundation.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Dan Rosenberg <drosenberg@...curity.com>,
	Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...omium.org>
Subject: Re: [PATCH] x86: make IDT read-only

On Wed, Apr 10, 2013 at 3:40 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Ingo Molnar <mingo@...nel.org> writes:
>
>> * Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>
>>> "H. Peter Anvin" <hpa@...or.com> writes:
>>>
>>> > On 04/08/2013 03:43 PM, Kees Cook wrote:
>>> >> This makes the IDT unconditionally read-only. This primarily removes
>>> >> the IDT from being a target for arbitrary memory write attacks. It has
>>> >> an added benefit of also not leaking (via the "sidt" instruction) the
>>> >> kernel base offset, if it has been relocated.
>>> >>
>>> >> Signed-off-by: Kees Cook <keescook@...omium.org>
>>> >> Cc: Eric Northup <digitaleric@...gle.com>
>>> >
>>> > Also, tglx: does this interfere with your per-cpu IDT efforts?
>>>
>>> Given that we don't change any IDT entries why would anyone want a
>>> per-cpu IDT?  The cache lines should easily be shared accross all
>>> processors.
>>
>> That's true iif they are cached.
>>
>> If not then it's a remote DRAM access cache miss for all CPUs except the node that
>> holds that memory.
>>
>>> Or are there some giant NUMA machines that trigger cache misses when accessing
>>> the IDT and the penalty for pulling the cache line across the NUMA fabric is
>>> prohibitive?
>>
>> IDT accesses for pure userspace execution are pretty rare. So we are not just
>> talking about huge NUMA machines here but about ordinary NUMA machines taking a
>> remote cache miss hit for the first IRQ or other IDT-accessing operation they do
>> after some cache-intense user-space processing.
>>
>> It's a small effect, but it exists and improving it would be
>> legitimate.
>
> If the effect is measurable I agree it is a legitimate optimization.  At
> one point there was a suggestion to make the code in the IDT vectors
> differ based on the which interrupt was registed.  While that can also
> reduce cache misses that can get hairy very quickly, and of course that
> would require read-write IDTs.

read-write IDT or GDT are fine: map them twice, once read+write, once
read-only.  Point the GDTR and IDTR at the read-only alias.

>
> My only practical concern with duplicating the IDT tables per cpu is (a)
> there are generic idt handlers that remain unduplicated reducing the
> benefit and this is essentially the same optimization as making the
> entire kernel text per cpu which last time it was examined was not an
> optimization worth making.  So I wonder if just a subset of the
> optimization is worth making.
>
> Eric
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