From: Jason Gunthorpe <jgg@nvidia.com>
To: Alistair Popple <apopple@nvidia.com>
Cc: linux-mm@kvack.org, nouveau@lists.freedesktop.org,
bskeggs@redhat.com, akpm@linux-foundation.org,
linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
kvm-ppc@vger.kernel.org, dri-devel@lists.freedesktop.org,
jhubbard@nvidia.com, rcampbell@nvidia.com, jglisse@redhat.com,
hch@infradead.org, daniel@ffwll.ch, willy@infradead.org,
Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v7 5/8] mm: Device exclusive memory access
Date: Thu, 1 Apr 2021 08:55:23 -0300 [thread overview]
Message-ID: <20210401115523.GZ1463678@nvidia.com> (raw)
In-Reply-To: <9244031.kThbcIvB84@nvdebian>
On Thu, Apr 01, 2021 at 01:20:05PM +1100, Alistair Popple wrote:
> On Thursday, 1 April 2021 11:48:13 AM AEDT Jason Gunthorpe wrote:
> > On Thu, Apr 01, 2021 at 11:45:57AM +1100, Alistair Popple wrote:
> > > On Thursday, 1 April 2021 12:46:04 AM AEDT Jason Gunthorpe wrote:
> > > > On Thu, Apr 01, 2021 at 12:27:52AM +1100, Alistair Popple wrote:
> > > > > On Thursday, 1 April 2021 12:18:54 AM AEDT Jason Gunthorpe wrote:
> > > > > > On Wed, Mar 31, 2021 at 11:59:28PM +1100, Alistair Popple wrote:
> > > > > >
> > > > > > > I guess that makes sense as the split could go either way at the
> > > > > > > moment but I should add a check to make sure this isn't used with
> > > > > > > pinned pages anyway.
> > > > > >
> > > > > > Is it possible to have a pinned page under one of these things? If I
> > > > > > pin it before you migrate it then it remains pinned but hidden under
> > > > > > the swap entry?
> > > > >
> > > > > At the moment yes. But I had planned (and this reminded me) to add a
> check
> > > to
> > > > > prevent marking pinned pages for exclusive access.
> > > >
> > > > How do you even do that without races with GUP fast?
> > >
> > > Unless I've missed something I think I've convinced myself it should be
> safe
> > > to do the pin check after make_device_exclusive() has replaced all the
> PTEs
> > > with exclusive entries.
> > >
> > > GUP fast sequence:
> > > 1. Read PTE
> > > 2. Pin page
> > > 3. Check PTE
> > > 4. if PTE changed -> unpin and fallback
> > >
> > > If make_device_exclusive() runs after (1) it will either succeed or see
> the
> > > pin from (2) and fail (as desired). GUP should always see the PTE change
> and
> > > fallback which will revoke the exclusive access.
> >
> > AFAICT the user can trigger fork at that instant and fork will try to
> > copy the desposited migration entry before it has been checked
>
> In that case the child will get a read-only exclusive entry and eventually a
> page copy via do_wp_page()
Having do_wp_page() do a copy is a security bug. We closed it with the
at-fork checks.
Jason
next prev parent reply other threads:[~2021-04-01 11:55 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-26 0:07 [PATCH v7 0/8] Add support for SVM atomics in Nouveau Alistair Popple
2021-03-26 0:07 ` [PATCH v7 1/8] mm: Remove special swap entry functions Alistair Popple
2021-03-30 18:38 ` Jason Gunthorpe
2021-03-26 0:07 ` [PATCH v7 2/8] mm/swapops: Rework swap entry manipulation code Alistair Popple
2021-03-26 0:08 ` [PATCH v7 3/8] mm/rmap: Split try_to_munlock from try_to_unmap Alistair Popple
2021-03-30 18:49 ` Jason Gunthorpe
2021-03-30 22:09 ` Alistair Popple
2021-03-30 22:16 ` Alistair Popple
2021-03-30 22:24 ` Jason Gunthorpe
2021-03-30 22:43 ` John Hubbard
2021-03-30 22:56 ` Alistair Popple
2021-03-31 3:56 ` John Hubbard
2021-03-31 4:09 ` John Hubbard
2021-03-31 4:15 ` Alistair Popple
2021-03-31 11:57 ` Jason Gunthorpe
2021-04-01 4:36 ` Alistair Popple
2021-04-01 19:21 ` Shakeel Butt
2021-03-26 0:08 ` [PATCH v7 4/8] mm/rmap: Split migration into its own function Alistair Popple
2021-03-26 0:08 ` [PATCH v7 5/8] mm: Device exclusive memory access Alistair Popple
2021-03-30 19:32 ` Jason Gunthorpe
2021-03-31 12:59 ` Alistair Popple
2021-03-31 13:18 ` Jason Gunthorpe
2021-03-31 13:27 ` Alistair Popple
2021-03-31 13:46 ` Jason Gunthorpe
2021-04-01 0:45 ` Alistair Popple
2021-04-01 0:48 ` Jason Gunthorpe
2021-04-01 2:20 ` Alistair Popple
2021-04-01 11:55 ` Jason Gunthorpe [this message]
2021-03-26 0:08 ` [PATCH v7 6/8] mm: Selftests for exclusive device memory Alistair Popple
2021-03-26 0:08 ` [PATCH v7 7/8] nouveau/svm: Refactor nouveau_range_fault Alistair Popple
2021-03-26 0:08 ` [PATCH v7 8/8] nouveau/svm: Implement atomic SVM access Alistair Popple
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