linux-kernel - Re: [PATCH v7 3/3] x86: Make the GDT remapping read-only on 64-bit

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Date:   Tue, 14 Mar 2017 15:43:01 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Thomas Garnier <thgarnie@...gle.com>, Pavel Machek <pavel@....cz>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, zijun_hu <zijun_hu@....com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Andy Lutomirski <luto@...capital.net>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>,
        Jiri Kosina <jikos@...nel.org>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...e.de>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Frederic.Weisbecker@...or.com
Subject: Re: [PATCH v7 3/3] x86: Make the GDT remapping read-only on 64-bit

<fweisbec@...il.com>,"Luis R . Rodriguez" <mcgrof@...nel.org>,Stanislaw Gruszka <sgruszka@...hat.com>,Peter Zijlstra <peterz@...radead.org>,Josh Poimboeuf <jpoimboe@...hat.com>,Vitaly Kuznetsov <vkuznets@...hat.com>,Tim Chen <tim.c.chen@...ux.intel.com>,Joerg Roedel <joro@...tes.org>,.TF-8?B?UmFkaW0gS3LEjW3DocWZ?From: hpa@...or.com
Message-ID: <550F6209-025A-45E2-84E2-F00A3771C0B1@...or.com>

On March 14, 2017 2:20:19 PM PDT, Thomas Garnier <thgarnie@...gle.com> wrote:
>On Tue, Mar 14, 2017 at 2:04 PM, Pavel Machek <pavel@....cz> wrote:
>> On Tue 2017-03-14 10:05:08, Thomas Garnier wrote:
>>> This patch makes the GDT remapped pages read-only to prevent
>corruption.
>>> This change is done only on 64-bit.
>>>
>>> The native_load_tr_desc function was adapted to correctly handle a
>>> read-only GDT. The LTR instruction always writes to the GDT TSS
>entry.
>>> This generates a page fault if the GDT is read-only. This change
>checks
>>> if the current GDT is a remap and swap GDTs as needed. This function
>was
>>> tested by booting multiple machines and checking hibernation works
>>> properly.
>>>
>>> KVM SVM and VMX were adapted to use the writeable GDT. On VMX, the
>>> per-cpu variable was removed for functions to fetch the original
>GDT.
>>> Instead of reloading the previous GDT, VMX will reload the fixmap
>GDT as
>>> expected. For testing, VMs were started and restored on multiple
>>> configurations.
>>>
>>> Signed-off-by: Thomas Garnier <thgarnie@...gle.com>
>>
>> Can we get the same change for 32-bit, too? Growing differences
>> between 32 and 64 bit are a bit of a problem...
>>                                                                 Pavel
>
>It was discussed on previous versions that 32-bit read-only support
>would create issues that why it was favor for 64-bit only right now.
>
>>
>> --
>> (english) http://www.livejournal.com/~pavelmachek
>> (cesky, pictures)
>http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

We can't make the GDT read-only on 32 bits since we use task switches for last-resort recovery.  64 bits has IST instead.
-- 
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