Re: [PATCH] vfio iommu type1: Bypass the vma permission check in vfio_pin_pages_remote()

[Stefan Hajnoczi]

On Wed, Dec 02, 2020 at 10:45:11AM -0500, Peter Xu wrote:
> On Wed, Dec 02, 2020 at 02:33:56PM +0000, Stefan Hajnoczi wrote:
> > On Wed, Nov 25, 2020 at 10:57:11AM -0500, Peter Xu wrote:
> > > On Wed, Nov 25, 2020 at 01:05:25AM +0000, Justin He wrote:
> > > > > I'd appreciate if you could explain why vfio needs to dma map some
> > > > > PROT_NONE
> > > > 
> > > > Virtiofs will map a PROT_NONE cache window region firstly, then remap the sub
> > > > region of that cache window with read or write permission. I guess this might
> > > > be an security concern. Just CC virtiofs expert Stefan to answer it more accurately.
> > > 
> > > Yep.  Since my previous sentence was cut off, I'll rephrase: I was thinking
> > > whether qemu can do vfio maps only until it remaps the PROT_NONE regions into
> > > PROT_READ|PROT_WRITE ones, rather than trying to map dma pages upon PROT_NONE.
> > 
> > Userspace processes sometimes use PROT_NONE to reserve virtual address
> > space. That way future mmap(NULL, ...) calls will not accidentally
> > allocate an address from the reserved range.
> > 
> > virtio-fs needs to do this because the DAX window mappings change at
> > runtime. Initially the entire DAX window is just reserved using
> > PROT_NONE. When it's time to mmap a portion of a file into the DAX
> > window an mmap(fixed_addr, ...) call will be made.
> 
> Yes I can understand the rational on why the region is reserved.  However IMHO
> the real question is why such reservation behavior should affect qemu memory
> layout, and even further to VFIO mappings.
> 
> Note that PROT_NONE should likely mean that there's no backing page at all in
> this case.  Since vfio will pin all the pages before mapping the DMAs, it also
> means that it's at least inefficient, because when we try to map all the
> PROT_NONE pages we'll try to fault in every single page of it, even if they may
> not ever be used.
> 
> So I still think this patch is not doing the right thing.  Instead we should
> somehow teach qemu that the virtiofs memory region should only be the size of
> enabled regions (with PROT_READ|PROT_WRITE), rather than the whole reserved
> PROT_NONE region.

virtio-fs was not implemented with IOMMUs in mind. The idea is just to
install a kvm.ko memory region that exposes the DAX window.

Perhaps we need to treat the DAX window like an IOMMU? That way the
virtio-fs code can send map/unmap notifications and hw/vfio/ can
propagate them to the host kernel.

Stefan

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