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

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 3 Dec 2020 10:43:22 -0500
Peter Xu <peterx@redhat.com> wrote:

> On Thu, Dec 03, 2020 at 11:20:02AM +0000, Stefan Hajnoczi wrote:
> > 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.  
> 
> Sounds right.  One more thing to mention is that we may need to avoid tearing
> down the whole old DMA region when resizing the PROT_READ|PROT_WRITE region
> into e.g. a bigger one to cover some of the previusly PROT_NONE part, as long
> as if the before-resizing region is still possible to be accessed from any
> hardware.  It smells like something David is working with virtio-mem, not sure
> whether there's any common infrastructure that could be shared.

Yes, very similar to his RamDiscardMgr which works on a granularity
basis.  vfio maps in granularity chunks and unmaps can span multiple
chunks.  This usage might be similar enough to use as-is.  Thanks,

Alex