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

["H. Peter Anvin" <hpa@xxxxxxxxx>]

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From: hpa@xxxxxxxxx
Message-ID: <550F6209-025A-45E2-84E2-F00A3771C0B1@xxxxxxxxx>

On March 14, 2017 2:20:19 PM PDT, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>On Tue, Mar 14, 2017 at 2:04 PM, Pavel Machek <pavel@xxxxxx> 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@xxxxxxxxxx>
>>
>> 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.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.

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