Re: [PATCH 11/13] mm: add unsafe_follow_pfn

[Daniel Vetter]

On Wed, Oct 7, 2020 at 9:00 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> On Wed, Oct 07, 2020 at 08:10:34PM +0200, Daniel Vetter wrote:
> > On Wed, Oct 7, 2020 at 7:36 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > >
> > > On Wed, Oct 07, 2020 at 06:44:24PM +0200, Daniel Vetter wrote:
> > > > Way back it was a reasonable assumptions that iomem mappings never
> > > > change the pfn range they point at. But this has changed:
> > > >
> > > > - gpu drivers dynamically manage their memory nowadays, invalidating
> > > > ptes with unmap_mapping_range when buffers get moved
> > > >
> > > > - contiguous dma allocations have moved from dedicated carvetouts to
> > > > cma regions. This means if we miss the unmap the pfn might contain
> > > > pagecache or anon memory (well anything allocated with GFP_MOVEABLE)
> > > >
> > > > - even /dev/mem now invalidates mappings when the kernel requests that
> > > > iomem region when CONFIG_IO_STRICT_DEVMEM is set, see 3234ac664a87
> > > > ("/dev/mem: Revoke mappings when a driver claims the region")
> > > >
> > > > Accessing pfns obtained from ptes without holding all the locks is
> > > > therefore no longer a good idea.
> > > >
> > > > Unfortunately there's some users where this is not fixable (like v4l
> > > > userptr of iomem mappings) or involves a pile of work (vfio type1
> > > > iommu). For now annotate these as unsafe and splat appropriately.
> > > >
> > > > This patch adds an unsafe_follow_pfn, which later patches will then
> > > > roll out to all appropriate places.
> > > >
> > > > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> > > > Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> > > > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > > > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > > Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> > > > Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > > > Cc: Jan Kara <jack@xxxxxxx>
> > > > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > > > Cc: linux-mm@xxxxxxxxx
> > > > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > > > Cc: linux-samsung-soc@xxxxxxxxxxxxxxx
> > > > Cc: linux-media@xxxxxxxxxxxxxxx
> > > > Cc: kvm@xxxxxxxxxxxxxxx
> > > >  include/linux/mm.h |  2 ++
> > > >  mm/memory.c        | 32 +++++++++++++++++++++++++++++++-
> > > >  mm/nommu.c         | 17 +++++++++++++++++
> > > >  security/Kconfig   | 13 +++++++++++++
> > > >  4 files changed, 63 insertions(+), 1 deletion(-)
> > >
> > > Makes sense to me.
> > >
> > > I wonder if we could change the original follow_pfn to require the
> > > ptep and then lockdep_assert_held() it against the page table lock?
> >
> > The safe variant with the pagetable lock is follow_pte_pmd. The only
> > way to make follow_pfn safe is if you have an mmu notifier and
> > corresponding retry logic. That is not covered by lockdep (it would
> > splat if we annotate the retry side), so I'm not sure how you'd check
> > for that?
>
> Right OK.
>
> > Checking for ptep lock doesn't work here, since the one leftover safe
> > user of this (kvm) doesn't need that at all, because it has the mmu
> > notifier.
>
> Ah, so a better name and/or function kdoc for follow_pfn is probably a
> good iead in this patch as well.

I did change that already to mention that you need an mmu notifier,
and that follow_pte_pmd respectively unsafe_follow_pfn are the
alternatives. Do you want more or something else here?

Note that I left the kerneldoc for the nommu.c case unchanged, since
without an mmu all bets are off anyway.

> > So I think we're as good as it gets, since I really have no idea how
> > to make sure follow_pfn callers do have an mmu notifier registered.
>
> Yah, can't be done. Most mmu notifier users should be using
> hmm_range_fault anyhow, kvm is really very special here.

We could pass an mmu notifier to follow_pfn and check that it has a
registration for vma->vm_mm, but that feels like overkill when kvm is
the only legit user for this.

> > I've followed the few other CONFIG_STRICT_FOO I've seen, which are all
> > explicit enables and default to "do not break uapi, damn the
> > (security) bugs". Which is I think how this should be done. It is in
> > the security section though, so hopefully competent distros will
> > enable this all.
>
> I thought the strict ones were more general and less clear security
> worries, not bugs like this.
>
> This is "allow a user triggerable use after free bug to exist in the
> kernel"

Since at most you get at GFP_MOVEABLE stuff I'm not sure you can use
this to pull the kernel over the table. Maybe best way is if you get a
gpu pagetable somehow into your pfn and then use that to access
abitrary stuff, but there's still an iommu. I think leveraging this is
going to be very tricky, and pretty much has to be device or driver
specific somehow.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch