linux-kernel - Re: [PATCH 3/3] mm: unexport follow

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Date:   Thu, 8 Apr 2021 13:40:59 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-media@...r.kernel.org,
        3pvd@...gle.com, Jann Horn <jannh@...gle.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 3/3] mm: unexport follow_pfn

On 08/04/21 12:05, Daniel Vetter wrote:
> On Mon, Mar 29, 2021 at 10:31:01AM -0300, Jason Gunthorpe wrote:
>> 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
>>
>> vfio and kvm
> 
> Hm I thought kvm is non-racy due to the mmu notifier catch races?

No, but the plan is indeed to have some struct for each page that uses 
follow_pfn and update it from the MMU notifiers.

Paolo

>>
>>> 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@...gle.com
>>> Cc: Jann Horn <jannh@...gle.com>
>>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>>> Cc: Jason Gunthorpe <jgg@...dia.com>
>>> Cc: Cornelia Huck <cohuck@...hat.com>
>>> Cc: Peter Xu <peterx@...hat.com>
>>> Cc: Alex Williamson <alex.williamson@...hat.com>
>>> Cc: linux-mm@...ck.org
>>> Cc: linux-arm-kernel@...ts.infradead.org
>>> Cc: linux-samsung-soc@...r.kernel.org
>>> Cc: linux-media@...r.kernel.org
>>> Cc: kvm@...r.kernel.org
>>> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
>>> ---
>>>   include/linux/mm.h |  2 --
>>>   mm/memory.c        | 26 +++++---------------------
>>>   mm/nommu.c         | 13 +------------
>>>   3 files changed, 6 insertions(+), 35 deletions(-)
>>
>> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> 
> Thanks for your r-b tags, I'll add them.
> -Daniel
> 
>>
>> Jason
>