35e351780fa9 ("fork: defer linking file vma until vma is fully initialized")
switched the ordering of vm_ops->open() and copy_page_range() on fork. This is a
bug for VFIO, because it causes two problems:

1. Because open() is called before copy_page_range(), the range can conceivably
   have unmapped 'holes' in it. This causes the code underneath untrack_pfn() to
   WARN.

2. More seriously, open() is trying to guarantee that the entire range is
   zapped, so any future accesses in the child will result in the VFIO fault
   handler being called. Because we copy_page_range() *after* open() (and
   therefore after zapping), this guarantee is violated.

We can't revert 35e351780fa9, because it fixes a real bug for hugetlbfs. The fix
is also not as simple as just reodering open() and copy_page_range(), as Miaohe
points out in [1]. So, although these patches are kind of large for stable, just
backport this refactoring which completely sidesteps the issue.

Note that patch 2 is the key one here which fixes the issue. Patch 1 is a
prerequisite required for patch 2 to build / work. This would almost be enough,
but we might see significantly regressed performance. Patch 3 fixes that up,
putting performance back on par with what it was before.

Note [1] also has a more full discussion justifying taking these backports.

I proposed the same backport for 6.9 [2], and now for 6.6. 6.6 is the oldest
kernel which needs the change: 35e351780fa9 was reverted for unrelated reasons
in 6.1, and was never backported to 5.15 or earlier.

[1]: https://lore.kernel.org/all/20240702042948.2629267-1-leah.rumancik@gmail.com/T/
[2]: https://lore.kernel.org/r/20240717213339.1921530-1-axelrasmussen@google.com

Alex Williamson (3):
  vfio: Create vfio_fs_type with inode per device
  vfio/pci: Use unmap_mapping_range()
  vfio/pci: Insert full vma on mmap'd MMIO fault

 drivers/vfio/device_cdev.c       |   7 +
 drivers/vfio/group.c             |   7 +
 drivers/vfio/pci/vfio_pci_core.c | 271 ++++++++-----------------------
 drivers/vfio/vfio_main.c         |  44 +++++
 include/linux/vfio.h             |   1 +
 include/linux/vfio_pci_core.h    |   2 -
 6 files changed, 125 insertions(+), 207 deletions(-)

--
2.45.2.993.g49e7a77208-goog

On Wed, 17 Jul 2024 15:24:26 -0700
Axel Rasmussen <axelrasmussen@google.com> wrote:

> 35e351780fa9 ("fork: defer linking file vma until vma is fully initialized")
> switched the ordering of vm_ops->open() and copy_page_range() on fork. This is a
> bug for VFIO, because it causes two problems:
> 
> 1. Because open() is called before copy_page_range(), the range can conceivably
>    have unmapped 'holes' in it. This causes the code underneath untrack_pfn() to
>    WARN.
> 
> 2. More seriously, open() is trying to guarantee that the entire range is
>    zapped, so any future accesses in the child will result in the VFIO fault
>    handler being called. Because we copy_page_range() *after* open() (and
>    therefore after zapping), this guarantee is violated.
> 
> We can't revert 35e351780fa9, because it fixes a real bug for hugetlbfs. The fix
> is also not as simple as just reodering open() and copy_page_range(), as Miaohe
> points out in [1]. So, although these patches are kind of large for stable, just
> backport this refactoring which completely sidesteps the issue.
> 
> Note that patch 2 is the key one here which fixes the issue. Patch 1 is a
> prerequisite required for patch 2 to build / work. This would almost be enough,
> but we might see significantly regressed performance. Patch 3 fixes that up,
> putting performance back on par with what it was before.
> 
> Note [1] also has a more full discussion justifying taking these backports.
> 
> I proposed the same backport for 6.9 [2], and now for 6.6. 6.6 is the oldest
> kernel which needs the change: 35e351780fa9 was reverted for unrelated reasons
> in 6.1, and was never backported to 5.15 or earlier.

AFAICT 35e351780fa9 was reverted in linux-6.6.y as well, so why isn't
this one a 4-part series concluding with a new backport of that commit?
I think without that, we don't need these in 6.6 either.  Thanks,

Alex

> 
> [1]: https://lore.kernel.org/all/20240702042948.2629267-1-leah.rumancik@gmail.com/T/
> [2]: https://lore.kernel.org/r/20240717213339.1921530-1-axelrasmussen@google.com
> 
> Alex Williamson (3):
>   vfio: Create vfio_fs_type with inode per device
>   vfio/pci: Use unmap_mapping_range()
>   vfio/pci: Insert full vma on mmap'd MMIO fault
> 
>  drivers/vfio/device_cdev.c       |   7 +
>  drivers/vfio/group.c             |   7 +
>  drivers/vfio/pci/vfio_pci_core.c | 271 ++++++++-----------------------
>  drivers/vfio/vfio_main.c         |  44 +++++
>  include/linux/vfio.h             |   1 +
>  include/linux/vfio_pci_core.h    |   2 -
>  6 files changed, 125 insertions(+), 207 deletions(-)
> 
> --
> 2.45.2.993.g49e7a77208-goog
>

On Wed, Jul 17, 2024 at 3:31 PM Alex Williamson
<alex.williamson@redhat.com> wrote:
>
> On Wed, 17 Jul 2024 15:24:26 -0700
> Axel Rasmussen <axelrasmussen@google.com> wrote:
>
> > 35e351780fa9 ("fork: defer linking file vma until vma is fully initialized")
> > switched the ordering of vm_ops->open() and copy_page_range() on fork. This is a
> > bug for VFIO, because it causes two problems:
> >
> > 1. Because open() is called before copy_page_range(), the range can conceivably
> >    have unmapped 'holes' in it. This causes the code underneath untrack_pfn() to
> >    WARN.
> >
> > 2. More seriously, open() is trying to guarantee that the entire range is
> >    zapped, so any future accesses in the child will result in the VFIO fault
> >    handler being called. Because we copy_page_range() *after* open() (and
> >    therefore after zapping), this guarantee is violated.
> >
> > We can't revert 35e351780fa9, because it fixes a real bug for hugetlbfs. The fix
> > is also not as simple as just reodering open() and copy_page_range(), as Miaohe
> > points out in [1]. So, although these patches are kind of large for stable, just
> > backport this refactoring which completely sidesteps the issue.
> >
> > Note that patch 2 is the key one here which fixes the issue. Patch 1 is a
> > prerequisite required for patch 2 to build / work. This would almost be enough,
> > but we might see significantly regressed performance. Patch 3 fixes that up,
> > putting performance back on par with what it was before.
> >
> > Note [1] also has a more full discussion justifying taking these backports.
> >
> > I proposed the same backport for 6.9 [2], and now for 6.6. 6.6 is the oldest
> > kernel which needs the change: 35e351780fa9 was reverted for unrelated reasons
> > in 6.1, and was never backported to 5.15 or earlier.
>
> AFAICT 35e351780fa9 was reverted in linux-6.6.y as well, so why isn't
> this one a 4-part series concluding with a new backport of that commit?
> I think without that, we don't need these in 6.6 either.  Thanks,

Ah! You are correct, I had failed to notice:

dd782da47 ("Revert "fork: defer linking file vma until vma is fully
initialized"")

in linux-6.6.y. So, please ignore this series for 6.6 then. :)

>
> Alex
>
> >
> > [1]: https://lore.kernel.org/all/20240702042948.2629267-1-leah.rumancik@gmail.com/T/
> > [2]: https://lore.kernel.org/r/20240717213339.1921530-1-axelrasmussen@google.com
> >
> > Alex Williamson (3):
> >   vfio: Create vfio_fs_type with inode per device
> >   vfio/pci: Use unmap_mapping_range()
> >   vfio/pci: Insert full vma on mmap'd MMIO fault
> >
> >  drivers/vfio/device_cdev.c       |   7 +
> >  drivers/vfio/group.c             |   7 +
> >  drivers/vfio/pci/vfio_pci_core.c | 271 ++++++++-----------------------
> >  drivers/vfio/vfio_main.c         |  44 +++++
> >  include/linux/vfio.h             |   1 +
> >  include/linux/vfio_pci_core.h    |   2 -
> >  6 files changed, 125 insertions(+), 207 deletions(-)
> >
> > --
> > 2.45.2.993.g49e7a77208-goog
> >
>