linux-kernel - Re: [PATCH v3 01/11] x86/boot: enumerate documentation for the x86 hardware

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Date:	Wed, 24 Feb 2016 17:29:28 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	X86 ML <x86@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Ingo Molnar <mingo@...nel.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	lguest@...ts.ozlabs.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v3 01/11] x86/boot: enumerate documentation for the x86 hardware_subarch

On Wed, Feb 24, 2016 at 5:18 PM, Luis R. Rodriguez <mcgrof@...nel.org> wrote:
>
> On Feb 24, 2016 8:40 AM, "Andy Lutomirski" <luto@...capital.net> wrote:
>>
>> On Feb 24, 2016 12:33 AM, "Ingo Molnar" <mingo@...nel.org> wrote:
>> >
>> > For hard coded platform quirks I'd suggest we add x86_platform.quirks
>> > flags. For
>> > example the F00F hack for Xen could be done via:
>> >
>> >         x86_platform.quirks.idt_remap = 0;
>> >
>>
>> Don't we unconditionally remap the IDT?  I think Kees did it for
>> general purpose hardening due to our complete inability to hide the
>> IDT address. I.e. I think we can remove the f00f condition entirely.
>>
>
> Kees can you confirm ?
>

No need.

        /*
         * Set the IDT descriptor to a fixed read-only location, so that the
         * "sidt" instruction will not leak the location of the kernel, and
         * to defend the IDT against arbitrary memory write vulnerabilities.
         * It will be reloaded in cpu_init() */
        __set_fixmap(FIX_RO_IDT, __pa_symbol(idt_table), PAGE_KERNEL_RO);
        idt_descr.address = fix_to_virt(FIX_RO_IDT);

IIUI this works around f00f as a side effect.  The only other thing
needed is the code that X86_BUG_F00F guards, which is responsible for
fixing up the error generated on attempted F00F exploitation from an
OOPS to a SIGILL.  I see no reason why that code couldn't be allowed
to run on even a PV guest on a F00F-affected CPU -- it would never
trigger anyway.