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Lorenzo Stoakes (ARM) 838bb60dc2 mm/mremap: fix locked_vm leak from MREMAP_DONTUNMAP self-merge
Patch series "mm/mremap: fix two issues with MREMAP_DONTUNMAP", v2.

The MREMAP_DONTUNMAP feature is highly unusual in that it permits mremap()
operations that keep the original VMA in place.

Historically this has led to a lot of bugs where non-obvious interactions
occur between existing mremap() operations and the original VMA.

Commit 397432cab1 ("mm/mremap: account mm->locked_vm correctly for
MREMAP_DONTUNMAP") fixed an accidentally introduced bug around
mm->locked_vm accounting, but this wasn't the only issue.

And thus history repeats itself, as it turns out that mm->locked_vm
accounting is broken by MREMAP_DONTUNMAP yet again by two further cases,
and has been broken ever since the feature was introduced.

Both relate to the fact that VMA_LOCKED_BIT is cleared on the source VMA
(it has to be as all page tables are moved):

1. If an unfaulted VMA_LOCKONFAULT_BIT anonymous VMA self-merges it
   clears the VMA_LOCKED_BIT flag and permanently leaks mm->locked_vm
   pages.

2. If a partial mremap() is performed on a locked VMA there is a leak equal
   to the number of pages not copied.

(Both for MREMAP_DONTUNMAP operations only)

Both issues can be fixed by treating the source range as distinct from the
destination range, which is the definition of what MREMAP_DONTUNMAP does
so is appropriate.

In case 1, simply disallow the self-merge, keeping adjacent source and
destination VMAs distinct.

In case 2, split the source range ahead of time if the VMA is mlock()'d,
so accounting is always correct.

Both changes were tested locally and confirmed to fix the issues.

For the purposes of a backport, the fixes are kept distinct, a follow-up
series can add self-tests.


This patch (of 2):

The MREMAP_DONTUNMAP feature is highly unusual in that it permits mremap()
operations that keep the original VMA in place.

Historically this has led to a lot of bugs where non-obvious interactions
occur between existing mremap() operations and the original VMA.

Fix another of these - self-merge.

Self-merge occurs when a VMA is moved in front of or behind itself and the
attributes of the VMA permit such a merge.

Practically this can only happen for unfaulted anonymous VMAs due to the
page offset equality requirement for merge:

                           |------------|
                           |            |
                           |            v
        |...........||-----------||...........|
        |           || unfaulted ||           |
        |...........||-----------||...........|
              ^            |
              |            |
              |------------|

This becomes problematic if the VMA is configured by the user to
mlock-on-fault, i.e. the VMA_LOCKED_BIT, VMA_LOCKONFAULT_BIT VMA flags are
set.

MREMAP_DONTUNMAP clears mlock flags for the source VMA and maintains them
for the destination VMA.

Self-merge makes this impossible (there is only one VMA) and incorrectly
clears the destination VMA's mlock flags.

This causes a leak in mm->locked_vm as clearing this flag does not
decrement the counter and the VMA no longer has VMA_LOCKED_BIT set so it
is not decremented on unmap.

Resolve this by simply disallowing a self-merge in this case - the source
and destination VMAs are kept distinct and then are able to have distinct
mlock() flags.

Update dontunmap_complete() to make the now-redundant self-merge check a
VM_WARN_ON_ONCE() instead to guard against future regressions.

Also update the VMA userland tests to reflect the change.

Link: https://lore.kernel.org/20260930-fix-dontunmap-partial-self-merge-v2-0-f388985a0f0a@kernel.org
Link: https://lore.kernel.org/20260930-fix-dontunmap-partial-self-merge-v2-1-f388985a0f0a@kernel.org
Fixes: e346b38130 ("mm/mremap: add MREMAP_DONTUNMAP to mremap()")
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>
Tested-by: Anirudh Srinivasan <asrinivasan@oss.tenstorrent.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: <stable@vger.kernel.org>
History 2026-10-06 16:44:17 -07:00
arch bpf-fixes 2026-10-02 12:59:32 -07:00
block block-7.3-20261002 2026-10-02 12:25:40 -07:00
certs treewide: fix indentation and whitespace in Kconfig files 2026-05-28 21:24:42 -07:00
crypto crypto: aes - Fix undesired override of some optimized AES modes 2026-09-30 08:38:06 -07:00
Documentation smb client fixes for v7.3-rc6 2026-10-02 14:04:11 -07:00
drivers pci-v7.3-fixes-3 2026-10-02 12:45:08 -07:00
fs smb client fixes for v7.3-rc6 2026-10-02 14:04:11 -07:00
include smb client fixes for v7.3-rc6 2026-10-02 14:04:11 -07:00
init Random number generator fixes for Linux 7.3-rc6. 2026-10-02 11:28:45 -07:00
io_uring io_uring-7.3-20261002 2026-10-02 12:17:24 -07:00
ipc mm.git review status for master..mm-nonmm-stable 2026-08-23 08:07:11 -07:00
kernel bpf-fixes 2026-10-02 12:59:32 -07:00
lib 14 hotfixes. 10 are cc:stable. 11 are for MM. 2026-09-14 08:59:09 -07:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
mm mm/mremap: fix locked_vm leak from MREMAP_DONTUNMAP self-merge 2026-10-06 16:44:17 -07:00
net netfilter pull request 26-09-30 2026-10-01 12:00:40 +02:00
rust rust: net: netlink: validate attribute length before casting to c_int 2026-09-29 12:28:03 +02:00
samples rust: samples: add missing newlines in rust_print_main 2026-09-01 02:20:22 +02:00
scripts hid-for-linus-2026092202 2026-09-22 10:30:17 -07:00
security ipe/stable-7.3 PR 20260925 2026-09-25 15:55:03 -07:00
sound ASoC: Fixes for v7.3 2026-10-02 14:45:01 +02:00
tools mm/mremap: fix locked_vm leak from MREMAP_DONTUNMAP self-merge 2026-10-06 16:44:17 -07:00
usr usr: Correct a spelling by changing a letter 2026-08-05 14:51:58 +02:00
virt KVM: WARN if vCPU creation is in-progress when locking all vCPUs 2026-09-26 00:49:49 -04:00
.clang-format drm/bridge: rename drm_for_each_bridge_in_chain_scoped() to drm_for_each_bridge_in_chain() 2026-07-14 17:55:51 +02:00
.clippy.toml rust: bump Clippy's MSRV and clean incompatible_msrv allows 2026-04-07 09:51:39 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig editorconfig: add rst extension 2026-01-26 19:07:09 -08:00
.get_maintainer.ignore .get_maintainer.ignore: add myself 2026-09-07 10:01:33 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore SPDX patches for 7.2-rc1 2026-06-22 12:06:22 -07:00
.mailmap MAINTAINERS: update Eric Dumazet's email address 2026-09-24 13:33:37 -07:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS Driver core fixes for 7.3-rc2 2026-09-05 11:59:05 -07:00
Kbuild checksyscalls: move instance functionality into generic code 2026-04-05 09:21:32 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS slab fixes for 7.3-rc5 2026-10-02 10:51:34 -07:00
Makefile Linux 7.3-rc5 2026-09-27 13:55:01 -07:00
README README: remove out of place emdashes 2026-07-15 10:51:29 -06:00

Linux kernel
============

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


Who Are You?
============

Find your role below:

* New Kernel Developer: Getting started with kernel development
* Academic Researcher: Studying kernel internals and architecture
* Security Expert: Hardening and vulnerability analysis
* Backport/Maintenance Engineer: Maintaining stable kernels
* System Administrator: Configuring and troubleshooting
* Maintainer: Leading subsystems and reviewing patches
* Hardware Vendor: Writing drivers for new hardware
* Distribution Maintainer: Packaging kernels for distros
* AI Coding Assistant: LLMs and AI-powered development tools


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
* Power Management: Documentation/power/index.rst

Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
* CVE Procedures: Documentation/process/cve.rst
* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

System Administrator
--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
* Tracing/Debugging: Documentation/trace/index.rst
* Performance Security: Documentation/admin-guide/perf-security.rst
* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
* Pull Requests: Documentation/maintainer/pull-requests.rst
* Managing Patches: Documentation/maintainer/modifying-patches.rst
* Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst
* Development Process: Documentation/process/maintainer-handbooks.rst
* Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

Write drivers and support new hardware:

* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst

AI Coding Assistant
-------------------

CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and
follow the AI coding assistants documentation before contributing to the Linux
kernel:

* Documentation/process/coding-assistants.rst

This documentation contains essential requirements about licensing, attribution,
and the Developer Certificate of Origin that all AI tools must comply with.


Communication and Support
=========================

* Mailing Lists: https://lore.kernel.org/
* IRC: #kernelnewbies on irc.oftc.net
* Bugzilla: https://bugzilla.kernel.org/
* MAINTAINERS file: Lists subsystem maintainers and mailing lists
* Email Clients: Documentation/process/email-clients.rst