linux-kernel - Re: [PATCH 6.6 0/3] Backport VFIO refactor to fix fork ordering bug

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Message-ID: <20240717163143.49b914cb.alex.williamson@redhat.com>
Date: Wed, 17 Jul 2024 16:31:43 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Axel Rasmussen <axelrasmussen@...gle.com>
Cc: stable@...r.kernel.org, Ankit Agrawal <ankita@...dia.com>, Eric Auger
 <eric.auger@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>, Kevin Tian
 <kevin.tian@...el.com>, Kunwu Chan <chentao@...inos.cn>, Leah Rumancik
 <leah.rumancik@...il.com>, Miaohe Lin <linmiaohe@...wei.com>, Stefan
 Hajnoczi <stefanha@...hat.com>, Yi Liu <yi.l.liu@...el.com>,
 kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6.6 0/3] Backport VFIO refactor to fix fork ordering bug

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