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

["H. Peter Anvin" <hpa@zytor.com>]

<fweisbec@gmail.com>,"Luis R . Rodriguez" <mcgrof@kernel.org>,Stanislaw Gruszka <sgruszka@redhat.com>,Peter Zijlstra <peterz@infradead.org>,Josh Poimboeuf <jpoimboe@redhat.com>,Vitaly Kuznetsov <vkuznets@redhat.com>,Tim Chen <tim.c.chen@linux.intel.com>,Joerg Roedel <joro@8bytes.org>,=?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,the arch/x86 maintainers <x86@kernel.org>,LKML <linux-kernel@vger.kernel.org>,linux-doc@vger.kernel.org,kasan-dev <kasan-dev@googlegroups.com>,Linux-MM <linux-mm@kvack.org>,Linux PM list <linux-pm@vger.kernel.org>,linux-efi@vger.kernel.org,xen-devel@lists.xenproject.org,lguest@lists.ozlabs.org,kvm list <kvm@vger.kernel.org>,Kernel Hardening <kernel-hardening@lists.openwall.com>
From: hpa@zytor.com
Message-ID: <550F6209-025A-45E2-84E2-F00A3771C0B1@zytor.com>

On March 14, 2017 2:20:19 PM PDT, Thomas Garnier <thgarnie@google.com> wrote:
>On Tue, Mar 14, 2017 at 2:04 PM, Pavel Machek <pavel@ucw.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@google.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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