Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address

[Michael S. Tsirkin]

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?

> Anyway just wanted to put the idea forward.
> 
> Cheers,
> Jérôme