From: John Hubbard <jhubbard@nvidia.com>
To: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Jérôme Glisse" <jglisse@redhat.com>,
linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
"Jan Kara" <jack@suse.cz>, "Pawel Osciak" <pawel@osciak.com>,
"KVM list" <kvm@vger.kernel.org>,
"Jason Gunthorpe" <jgg@ziepe.ca>,
"Mauro Carvalho Chehab" <mchehab@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
"DRI Development" <dri-devel@lists.freedesktop.org>,
"Tomasz Figa" <tfiga@chromium.org>,
"Linux MM" <linux-mm@kvack.org>,
"Kyungmin Park" <kyungmin.park@samsung.com>,
"Daniel Vetter" <daniel.vetter@intel.com>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Marek Szyprowski" <m.szyprowski@samsung.com>,
"Dan Williams" <dan.j.williams@intel.com>,
"Linux ARM" <linux-arm-kernel@lists.infradead.org>,
"open list:DMA BUFFER SHARING FRAMEWORK"
<linux-media@vger.kernel.org>
Subject: Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM
Date: Sun, 1 Nov 2020 13:13:07 -0800 [thread overview]
Message-ID: <7f29a42a-c408-525d-90b7-ef3c12b5826c@nvidia.com> (raw)
In-Reply-To: <CAKMK7uFYDSqnNp_xpohzCEidw_iLufNSoX4v55sNZj-nwTckSg@mail.gmail.com>
On 11/1/20 2:30 AM, Daniel Vetter wrote:
> On Sun, Nov 1, 2020 at 6:22 AM John Hubbard <jhubbard@nvidia.com> wrote:
>>
>> On 10/31/20 7:45 AM, Daniel Vetter wrote:
>>> On Sat, Oct 31, 2020 at 3:55 AM John Hubbard <jhubbard@nvidia.com> wrote:
>>>> On 10/30/20 3:08 AM, Daniel Vetter wrote:
>> ...
>>>> By removing this check from this location, and changing from
>>>> pin_user_pages_locked() to pin_user_pages_fast(), I *think* we end up
>>>> losing the check entirely. Is that intended? If so it could use a comment
>>>> somewhere to explain why.
>>>
>>> Yeah this wasn't intentional. I think I needed to drop the _locked
>>> version to prep for FOLL_LONGTERM, and figured _fast is always better.
>>> But I didn't realize that _fast doesn't have the vma checks, gup.c got
>>> me a bit confused.
>>
>> Actually, I thought that the change to _fast was a very nice touch, btw.
>>
>>>
>>> I'll remedy this in all the patches where this applies (because a
>>> VM_IO | VM_PFNMAP can point at struct page backed memory, and that
>>> exact use-case is what we want to stop with the unsafe_follow_pfn work
>>> since it wreaks things like cma or security).
>>>
>>> Aside: I do wonder whether the lack for that check isn't a problem.
>>> VM_IO | VM_PFNMAP generally means driver managed, which means the
>>> driver isn't going to consult the page pin count or anything like that
>>> (at least not necessarily) when revoking or moving that memory, since
>>> we're assuming it's totally under driver control. So if pup_fast can
>>> get into such a mapping, we might have a problem.
>>> -Daniel
>>>
>>
>> Yes. I don't know why that check is missing from the _fast path.
>> Probably just an oversight, seeing as how it's in the slow path. Maybe
>> the appropriate response here is to add a separate patch that adds the
>> check.
>>
>> I wonder if I'm overlooking something, but it certainly seems correct to
>> do that.
>
> You'll need the mmap_sem to get at the vma to be able to do this
> check. If you add that to _fast, you made it as fast as the slow one.
Arggh, yes of course. Strike that, please. :)
> Plus there's _fast_only due to locking recurion issues in fast-paths
> (I assume, I didn't check all the callers).
>
> I'm just wondering whether we have a bug somewhere with device
> drivers. For CMA regions we always check in try_grab_page, but for dax
OK, so here you're talking about a different bug than the VM_IO | VM_PFNMAP
pages, I think. This is about the "FOLL_LONGTERM + CMA + gup/pup _fast"
combination that is not allowed, right?
For that: try_grab_page() doesn't check anything, but try_grab_compound_head()
does, but only for pup_fast, not gup_fast. That was added by commit
df3a0a21b698d ("mm/gup: fix omission of check on FOLL_LONGTERM in gup fast
path") in April.
I recall that the patch was just plugging a very specific hole, as opposed
to locking down the API against mistakes or confused callers. And it does
seem that there are some holes.
> I'm not seeing where the checks in the _fast fastpaths are, and that
> all still leaves random device driver mappings behind which aren't
> backed by CMA but still point to something with a struct page behind
> it. I'm probably just missing something, but no idea what.
> -Daniel
>
Certainly we've established that we can't check VMA flags by that time,
so I'm not sure that there is much we can check by the time we get to
gup/pup _fast. Seems like the device drivers have to avoid calling _fast
with pages that live in VM_IO | VM_PFNMAP, by design, right? Or maybe
you're talking about CMA checks only?
thanks,
--
John Hubbard
NVIDIA
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next prev parent reply other threads:[~2020-11-01 21:13 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-30 10:08 [PATCH v5 00/15] follow_pfn and other iomap races Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 01/15] drm/exynos: Stop using frame_vector helpers Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 02/15] drm/exynos: Use FOLL_LONGTERM for g2d cmdlists Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 03/15] misc/habana: Stop using frame_vector helpers Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 04/15] misc/habana: Use FOLL_LONGTERM for userptr Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM Daniel Vetter
2020-10-30 14:11 ` Tomasz Figa
2020-10-30 14:37 ` Daniel Vetter
2020-11-02 18:19 ` Tomasz Figa
2020-10-31 2:55 ` John Hubbard
2020-10-31 14:45 ` Daniel Vetter
2020-11-01 5:22 ` John Hubbard
2020-11-01 10:30 ` Daniel Vetter
2020-11-01 21:13 ` John Hubbard [this message]
2020-11-01 22:50 ` Daniel Vetter
2020-11-04 14:00 ` Jason Gunthorpe
2020-11-04 15:54 ` Daniel Vetter
[not found] ` <20201104162125.GA13007@infradead.org>
2020-11-04 16:26 ` Daniel Vetter
[not found] ` <20201104163758.GA17425@infradead.org>
[not found] ` <20201104164119.GA18218@infradead.org>
2020-11-04 18:17 ` Jason Gunthorpe
2020-11-04 18:44 ` John Hubbard
2020-11-04 19:02 ` Jason Gunthorpe
2020-11-05 9:25 ` Daniel Vetter
2020-11-05 12:49 ` Jason Gunthorpe
2020-11-06 4:08 ` John Hubbard
2020-11-06 10:01 ` Daniel Vetter
2020-11-06 10:27 ` Daniel Vetter
2020-11-06 12:55 ` Jason Gunthorpe
2020-11-09 8:44 ` Thomas Hellström
2020-11-09 20:19 ` Jason Gunthorpe
2020-11-06 12:58 ` Jason Gunthorpe
2020-10-30 10:08 ` [PATCH v5 06/15] media: videobuf2: Move frame_vector into media subsystem Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 07/15] mm: Close race in generic_access_phys Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 08/15] mm: Add unsafe_follow_pfn Daniel Vetter
[not found] ` <20201102072931.GA16419@infradead.org>
2020-11-02 12:56 ` Daniel Vetter
2020-11-02 13:01 ` Jason Gunthorpe
2020-11-02 13:23 ` Daniel Vetter
2020-11-02 15:52 ` Jason Gunthorpe
2020-11-02 16:42 ` Daniel Vetter
2020-11-02 17:16 ` Jason Gunthorpe
2020-10-30 10:08 ` [PATCH v5 09/15] media/videbuf1|2: Mark follow_pfn usage as unsafe Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 10/15] vfio/type1: Mark follow_pfn " Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 11/15] PCI: Obey iomem restrictions for procfs mmap Daniel Vetter
2020-11-03 21:28 ` Bjorn Helgaas
2020-11-03 22:09 ` Dan Williams
2020-11-04 8:44 ` Daniel Vetter
2020-11-04 16:50 ` Bjorn Helgaas
2020-11-04 20:12 ` Dan Williams
2020-11-05 9:34 ` Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 12/15] /dev/mem: Only set filp->f_mapping Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 13/15] resource: Move devmem revoke code to resource framework Daniel Vetter
2020-10-31 6:36 ` John Hubbard
2020-10-31 14:40 ` Daniel Vetter
2020-11-03 6:06 ` [resource] 22b17dc667: Kernel panic - not syncing: Fatal exception lkp
2020-11-03 6:15 ` John Hubbard
2020-11-03 10:10 ` Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 14/15] sysfs: Support zapping of binary attr mmaps Daniel Vetter
2020-10-30 10:08 ` [PATCH v5 15/15] PCI: Revoke mappings like devmem Daniel Vetter
2020-10-30 19:22 ` Dan Williams
2020-11-03 21:30 ` Bjorn Helgaas
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