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

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Message-ID: <20240717222429.2011540-1-axelrasmussen@google.com>
Date: Wed, 17 Jul 2024 15:24:26 -0700
From: Axel Rasmussen <axelrasmussen@...gle.com>
To: stable@...r.kernel.org
Cc: Alex Williamson <alex.williamson@...hat.com>, 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, 
	Axel Rasmussen <axelrasmussen@...gle.com>
Subject: [PATCH 6.6 0/3] Backport VFIO refactor to fix fork ordering bug

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