linux-kernel - Re: [PATCH] vfio iommu type1: Bypass the vma permission check in vfio_pin_pages

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Message-ID: <20201202143356.GK655829@stefanha-x1.localdomain>
Date:   Wed, 2 Dec 2020 14:33:56 +0000
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Justin He <Justin.He@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vfio iommu type1: Bypass the vma permission check in
 vfio_pin_pages_remote()

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.

Stefan

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