Linux Virtualization - Mar 2019 - [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address

On Thu, Mar 07, 2019 at 10:43:12PM -0500, Michael S. Tsirkin
wrote:
> On Thu, Mar 07, 2019 at 10:40:53PM -0500, Jerome Glisse wrote:
> > On Thu, Mar 07, 2019 at 10:16:00PM -0500, Michael S. Tsirkin wrote:
> > > On Thu, Mar 07, 2019 at 09:55:39PM -0500, Jerome Glisse wrote:
> > > > On Thu, Mar 07, 2019 at 09:21:03PM -0500, Michael S. Tsirkin
wrote:
> > > > > On Thu, Mar 07, 2019 at 02:17:20PM -0500, Jerome Glisse
wrote:
> > > > > > > It's because of all these issues that I
preferred just accessing
> > > > > > > userspace memory and handling faults.
Unfortunately there does not
> > > > > > > appear to exist an API that whitelists a
specific driver along the lines
> > > > > > > of "I checked this code for speculative
info leaks, don't add barriers
> > > > > > > on data path please".
> > > > > > 
> > > > > > Maybe it would be better to explore adding such
helper then remapping
> > > > > > page into kernel address space ?
> > > > > 
> > > > > I explored it a bit (see e.g. thread around:
"__get_user slower than
> > > > > get_user") and I can tell you it's not trivial
given the issue is around
> > > > > security.  So in practice it does not seem fair to keep
a significant
> > > > > optimization out of kernel because *maybe* we can do it
differently even
> > > > > better :)
> > > > 
> > > > Maybe a slightly different approach between this patchset
and other
> > > > copy user API would work here. What you want really is
something like
> > > > a temporary mlock on a range of memory so that it is safe
for the
> > > > kernel to access range of userspace virtual address ie page
are
> > > > present and with proper permission hence there can be no
page fault
> > > > while you are accessing thing from kernel context.
> > > > 
> > > > So you can have like a range structure and mmu notifier.
When you
> > > > lock the range you block mmu notifier to allow your code to
work on
> > > > the userspace VA safely. Once you are done you unlock and
let the
> > > > mmu notifier go on. It is pretty much exactly this patchset
except
> > > > that you remove all the kernel vmap code. A nice thing about
that
> > > > is that you do not need to worry about calling set page
dirty it
> > > > will already be handle by the userspace VA pte. It also use
less
> > > > memory than when you have kernel vmap.
> > > > 
> > > > This idea might be defeated by security feature where the
kernel is
> > > > running in its own address space without the userspace
address
> > > > space present.
> > > 
> > > Like smap?
> > 
> > Yes like smap but also other newer changes, with similar effect, since
> > the spectre drama.
> > 
> > Cheers,
> > J?r?me
> 
> Sorry do you mean meltdown and kpti?
Yes all that and similar thing. I do not have the full list in my head.

Cheers,
J?r?me

On 2019/3/8 ??11:45, Jerome Glisse wrote:
> On Thu, Mar 07, 2019 at 10:43:12PM -0500, Michael S. Tsirkin wrote:
>> On Thu, Mar 07, 2019 at 10:40:53PM -0500, Jerome Glisse wrote:
>>> On Thu, Mar 07, 2019 at 10:16:00PM -0500, Michael S. Tsirkin wrote:
>>>> On Thu, Mar 07, 2019 at 09:55:39PM -0500, Jerome Glisse wrote:
>>>>> On Thu, Mar 07, 2019 at 09:21:03PM -0500, Michael S.
Tsirkin wrote:
>>>>>> On Thu, Mar 07, 2019 at 02:17:20PM -0500, Jerome Glisse
wrote:
>>>>>>>> It's because of all these issues that I
preferred just accessing
>>>>>>>> userspace memory and handling faults.
Unfortunately there does not
>>>>>>>> appear to exist an API that whitelists a
specific driver along the lines
>>>>>>>> of "I checked this code for speculative
info leaks, don't add barriers
>>>>>>>> on data path please".
>>>>>>> Maybe it would be better to explore adding such
helper then remapping
>>>>>>> page into kernel address space ?
>>>>>> I explored it a bit (see e.g. thread around:
"__get_user slower than
>>>>>> get_user") and I can tell you it's not trivial
given the issue is around
>>>>>> security.  So in practice it does not seem fair to keep
a significant
>>>>>> optimization out of kernel because *maybe* we can do it
differently even
>>>>>> better :)
>>>>> Maybe a slightly different approach between this patchset
and other
>>>>> copy user API would work here. What you want really is
something like
>>>>> a temporary mlock on a range of memory so that it is safe
for the
>>>>> kernel to access range of userspace virtual address ie page
are
>>>>> present and with proper permission hence there can be no
page fault
>>>>> while you are accessing thing from kernel context.
>>>>>
>>>>> So you can have like a range structure and mmu notifier.
When you
>>>>> lock the range you block mmu notifier to allow your code to
work on
>>>>> the userspace VA safely. Once you are done you unlock and
let the
>>>>> mmu notifier go on. It is pretty much exactly this patchset
except
>>>>> that you remove all the kernel vmap code. A nice thing
about that
>>>>> is that you do not need to worry about calling set page
dirty it
>>>>> will already be handle by the userspace VA pte. It also use
less
>>>>> memory than when you have kernel vmap.
>>>>>
>>>>> This idea might be defeated by security feature where the
kernel is
>>>>> running in its own address space without the userspace
address
>>>>> space present.
>>>> Like smap?
>>> Yes like smap but also other newer changes, with similar effect,
since
>>> the spectre drama.
>>>
>>> Cheers,
>>> J?r?me
>> Sorry do you mean meltdown and kpti?
> Yes all that and similar thing. I do not have the full list in my head.
>
> Cheers,
> J?r?me

Yes, address space of kernel its own is the main motivation of using 
vmap here.

Thanks