linux-kernel - [PATCH v2 0/4] mm: Avoid sharing high VMA flag bits

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Message-ID: <20250507131000.1204175-1-revest@chromium.org>
Date: Wed,  7 May 2025 15:09:56 +0200
From: Florent Revest <revest@...omium.org>
To: linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Cc: catalin.marinas@....com,
	will@...nel.org,
	tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com,
	x86@...nel.org,
	hpa@...or.com,
	akpm@...ux-foundation.org,
	broonie@...nel.org,
	thiago.bauermann@...aro.org,
	jackmanb@...gle.com,
	Florent Revest <revest@...omium.org>
Subject: [PATCH v2 0/4] mm: Avoid sharing high VMA flag bits

While staring at include/linux/mm.h, I was wondering why VM_UFFD_MINOR and
VM_SHADOW_STACK share the same bit on arm64. I think I gained enough confidence
now to call it a bug.

The first patch of this series is a straightforward attempt at fixing this
specific bug by changing the bit used by VM_UFFD_MINOR. I cc-ed stable on that
one and I expect it to not be all too controversial.

The rest of the series however is a more zealous refactoring and likely to be
more contentious... :) Since this bug looks like a near miss which could have
been quite severe in terms of security, I think it's worth trying to simplify
the high VMA flag bits code. I tried to consolidate around the current usage of
VM_HIGH_ARCH_* macros but I'm not sure if this is the preferred approach here. I
really don't feel strongly about those refactorings so this is more of a
platform for discussion for people with more mm background, I'll be more than
happy to respin a v2!

This series applies on v6.15-rc5.

v1 -> v2:
- Fixed a couple of typos in patch 4
- Added Mark Brown's Reviewed-by tag

Florent Revest (4):
  mm: fix VM_UFFD_MINOR == VM_SHADOW_STACK on USERFAULTFD=y &&
    ARM64_GCS=y
  mm: remove CONFIG_ARCH_USES_HIGH_VMA_FLAGS
  mm: use VM_HIGH_ARCH_* macros consistently
  mm: consolidate VM_HIGH_ARCH_* macros into parametric macros

 arch/arm64/Kconfig   |  3 ---
 arch/powerpc/Kconfig |  1 -
 arch/x86/Kconfig     |  2 --
 include/linux/mm.h   | 51 ++++++++++++++++----------------------------
 mm/Kconfig           |  2 --
 5 files changed, 18 insertions(+), 41 deletions(-)

-- 
2.49.0.987.g0cc8ee98dc-goog