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Date:   Wed, 2 Dec 2020 14:33:56 +0000
From:   Stefan Hajnoczi <stefanha@redhat.com>
To:     Peter Xu <peterx@redhat.com>
Cc:     Justin He <Justin.He@arm.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] vfio iommu type1: Bypass the vma permission check in
 vfio_pin_pages_remote()
Message-ID: <20201202143356.GK655829@stefanha-x1.localdomain>
References: <20201119142737.17574-1-justin.he@arm.com>
 <20201124181228.GA276043@xz-x1>
 <AM6PR08MB32245E7F990955395B44CE6BF7FA0@AM6PR08MB3224.eurprd08.prod.outlook.com>
 <20201125155711.GA6489@xz-x1>
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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
> >=20
> > Virtiofs will map a PROT_NONE cache window region firstly, then remap t=
he 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 mor=
e accurately.
>=20
> Yep.  Since my previous sentence was cut off, I'll rephrase: I was thinki=
ng
> 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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