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From: Jason Gunthorpe <jgg@nvidia.com>
To: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: DRI Development <dri-devel@lists.freedesktop.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kvm@vger.kernel.org, linux-mm@kvack.org,
	linux-arm-kernel@lists.infradead.org,
	linux-samsung-soc@vger.kernel.org, linux-media@vger.kernel.org,
	3pvd@google.com, Jann Horn <jannh@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Cornelia Huck <cohuck@redhat.com>, Peter Xu <peterx@redhat.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH 3/3] mm: unexport follow_pfn
Date: Wed, 24 Mar 2021 09:52:11 -0300	[thread overview]
Message-ID: <20210324125211.GA2356281@nvidia.com> (raw)
In-Reply-To: <20210316153303.3216674-4-daniel.vetter@ffwll.ch>

On Tue, Mar 16, 2021 at 04:33:03PM +0100, Daniel Vetter wrote:
> Both kvm (in bd2fae8da794 ("KVM: do not assume PTE is writable after
> follow_pfn")) and vfio (in 07956b6269d3 ("vfio/type1: Use
> follow_pte()")) have lost their callsites of follow_pfn(). All the
> other ones have been switched over to unsafe_follow_pfn because they
> cannot be fixed without breaking userspace api.
> 
> Argueably the vfio code is still racy, but that's kinda a bigger
> picture. But since it does leak the pte beyond where it drops the pt
> lock, without anything else like an mmu notifier guaranteeing
> coherence, the problem is at least clearly visible in the vfio code.
> So good enough with me.
> 
> I've decided to keep the explanation that after dropping the pt lock
> you must have an mmu notifier if you keep using the pte somehow by
> adjusting it and moving it into the kerneldoc for the new follow_pte()
> function.
> 
> Cc: 3pvd@google.com
> Cc: Jann Horn <jannh@google.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Jason Gunthorpe <jgg@nvidia.com>
> Cc: Cornelia Huck <cohuck@redhat.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Alex Williamson <alex.williamson@redhat.com>
> Cc: linux-mm@kvack.org
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-samsung-soc@vger.kernel.org
> Cc: linux-media@vger.kernel.org
> Cc: kvm@vger.kernel.org
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>  include/linux/mm.h |  2 --
>  mm/memory.c        | 26 +++++---------------------
>  mm/nommu.c         | 13 +------------
>  3 files changed, 6 insertions(+), 35 deletions(-)

I think this is the right thing to do.

Alex is working on fixing VFIO and while kvm is still racy using
follow pte, I think they are working on it too?

Jason


  reply	other threads:[~2021-03-24 12:52 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-16 15:33 [PATCH 0/3] switch to unsafe_follow_pfn Daniel Vetter
2021-03-16 15:33 ` [PATCH 1/3] mm: Add unsafe_follow_pfn Daniel Vetter
2021-03-29 13:29   ` Jason Gunthorpe
2021-03-16 15:33 ` [PATCH 2/3] media/videobuf1|2: Mark follow_pfn usage as unsafe Daniel Vetter
2021-03-16 15:45   ` Christoph Hellwig
2021-03-16 15:52     ` Daniel Vetter
2021-03-17  7:22       ` Christoph Hellwig
2021-03-17  8:04         ` Daniel Vetter
2021-03-16 15:33 ` [PATCH 3/3] mm: unexport follow_pfn Daniel Vetter
2021-03-24 12:52   ` Jason Gunthorpe [this message]
2021-03-24 14:31     ` Paolo Bonzini
2021-03-24 19:17     ` Daniel Vetter
2021-03-25 21:33       ` Daniel Vetter
2021-03-29 13:31   ` Jason Gunthorpe
2021-04-08 10:05     ` Daniel Vetter
2021-04-08 11:40       ` Paolo Bonzini
2021-04-08 11:44         ` Daniel Vetter

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