[PATCH v2 0/3] arm64/mm: Enable userfaultfd write-protect

Ryan Roberts
2024-04-29T14:02:21Z

Hi All,

This series adds uffd write-protect support for arm64.

Previous attempts to add uffd-wp (and soft-dirty) have failed because of a
perceived lack of available PTE SW bits. However it actually turns out that
there are 2 available but they are hidden. PTE_PROT_NONE was previously
occupying a SW bit, but can be moved, freeing up the SW bit. Bit 63 is marked as
"IGNORED" in the Arm ARM, but it does not currently indicate "reserved for SW
use" like it does for the other SW bits. I've confirmed with the spec owner that
this is an oversight; the bit is intended to be reserved for SW use and the spec
will clarify this in a future update.

So now we have two spare bits; patch 3 enables uffd-wp on arm64, using the SW
bit freed up by moving PTE_PROT_NONE. This leaves bit 63 spare for future use
(e.g. soft-dirty - see RFC at [2] - or some other usage).

---

This applies on top of v6.9-rc5.

All mm selftests involving uffd-wp now run. However, this work exposed a bug in
core-mm that was leading to some test uffd-wp failures in the pagemap_ioctl
test. The fix for that is posted at [3]. With the fix applied, all the uffd-wp
tests pass and no other selftest regressions are observed.

Changes since v1 [1]
====================

patch 1 & 2 (was patch 1):
- generalized PMD_PRESENT_INVALID into PTE_INVALID
- removed explicit PTE_PROT_NONE bit

patch 3 (was patch 2):
- collected R-b/A-b from Peter and Catalin - thanks!

[1] https://lore.kernel.org/linux-arm-kernel/20240424111017.3160195-1-ryan.roberts@arm.com/
[2] https://lore.kernel.org/all/20240419074344.2643212-1-ryan.roberts@arm.com/
[3] https://lore.kernel.org/all/20240429114104.182890-1-ryan.roberts@arm.com/

Thanks,
Ryan

Ryan Roberts (3):
arm64/mm: Refactor PMD_PRESENT_INVALID and PTE_PROT_NONE bits
arm64/mm: Move PTE_INVALID to overlay PTE_NS
arm64/mm: Add uffd write-protect support

arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/pgtable-hwdef.h | 1 +
arch/arm64/include/asm/pgtable-prot.h | 17 ++---
arch/arm64/include/asm/pgtable.h | 88 +++++++++++++++++++-------
4 files changed, 75 insertions(+), 32 deletions(-)

--
2.25.1