linux-kernel - [RFC PATCH 0/4] iommu: Add IOMMU_DEBUG

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Message-ID: <20251003173229.1533640-1-smostafa@google.com>
Date: Fri,  3 Oct 2025 17:32:25 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: linux-mm@...ck.org, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org
Cc: corbet@....net, joro@...tes.org, will@...nel.org, robin.murphy@....com, 
	akpm@...ux-foundation.org, vbabka@...e.cz, surenb@...gle.com, mhocko@...e.com, 
	jackmanb@...gle.com, hannes@...xchg.org, ziy@...dia.com, david@...hat.com, 
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, rppt@...nel.org, 
	Mostafa Saleh <smostafa@...gle.com>
Subject: [RFC PATCH 0/4] iommu: Add IOMMU_DEBUG_PAGEALLOC sanitizer

Overview
--------
This patch series introduces a new debugging feature,
IOMMU_DEBUG_PAGEALLOC, designed to catch DMA use-after-free bugs
and IOMMU mapping leaks from buggy drivers.

The kernel has powerful sanitizers like KASAN and DEBUG_PAGEALLOC
for catching CPU-side memory corruption. However, there is limited
runtime sanitization for DMA mappings managed by the IOMMU. A buggy
driver can free a page while it is still mapped for DMA, leading to
memory corruption or use-after-free vulnerabilities when that page is
reallocated and used for a different purpose.

Inspired by DEBUG_PAGEALLOC, this sanitizer tracks IOMMU mappings on a
per-page basis, as it’s not possible to unmap the pages, because it
requires to lock and walk all domains on every kernel free, instead we
rely on page_ext to add an IOMMU-specific mapping reference count for
each page.
And on each page allocated/freed from the kernel we simply check the
count and WARN if it is not zero.

Concurrency
-----------
By design this check is racy where one caller can map pages just after
the check, which can lead to false negatives.
In my opinion this is acceptable for sanitizers (for ex KCSAN have
that property).
Otherwise we have to implement locks in iommu_map/unmap for all domains
which is not favourable even for a debug feature.
The sanitizer only guarantees that the refcount itself doesn’t get
corrupted using atomics. And there are no false positives.

CPU vs IOMMU Page Size
----------------------
IOMMUs can use different page sizes and which can be non-homogeneous;
not even all of them have the same page size.

To solve this, the refcount is always incremented and decremented in
units of the smallest page size supported by the IOMMU domain. This
ensures the accounting remains consistent regardless of the size of
the map or unmap operation, otherwise double counting can happen.

Testing & Performance
---------------------
This was tested on Morello with Arm64 + SMMUv3
Also I booted RockPi-4b with Rockchip IOMMU.
Did some tests on Qemu including different SMMUv3/CPU page size (arm64).

I also ran dma_map_benchmark on Morello:

echo dma_map_benchmark > /sys/bus/pci/devices/0000\:06\:00.0/driver_override
echo 0000:06:00.0 >  /sys/bus/pci/devices/0000\:06\:00.0/driver/unbind
echo 0000:06:00.0 > /sys/bus/pci/drivers/dma_map_benchmark/bind
./dma_map_bechmark -t $threads -g $nr_pages

CONFIG refers to "CONFIG_IOMMU_DEBUG_PAGEALLOC"
cmdline refer to "iommu.debug_pagealloc"
Numbers are (map latency)/(unmap latency), lower is better.

			CONFIG=n    CONFIG=y    CONFIG=y
			            cmdline=0   cmdline=1
4K - 1 thread		0.1/0.6     0.1/0.6     0.1/0.7
4K - 4 threads		0.1/1.0     0.1/1.1     0.1/1.1
1M - 1 thread		0.8/21.2    0.8/21.2    5.6/42.5
1M - 4 threads		1.1/46.3    1.1/46.1    5.9/45.5

Thanks,
Mostafa

Mostafa Saleh (4):
  drivers/iommu: Add page_ext for IOMMU_DEBUG_PAGEALLOC
  drivers/iommu: Add calls for iommu debug
  drivers/iommu-debug: Track IOMMU pages
  drivers/iommu-debug: Check state of mapped/unmapped kernel memory

 .../admin-guide/kernel-parameters.txt         |   6 +
 drivers/iommu/Kconfig                         |  14 ++
 drivers/iommu/Makefile                        |   1 +
 drivers/iommu/iommu-debug.c                   | 160 ++++++++++++++++++
 drivers/iommu/iommu.c                         |  21 ++-
 include/linux/iommu-debug.h                   |  24 +++
 include/linux/mm.h                            |   7 +
 mm/page_ext.c                                 |   4 +
 8 files changed, 235 insertions(+), 2 deletions(-)
 create mode 100644 drivers/iommu/iommu-debug.c
 create mode 100644 include/linux/iommu-debug.h

-- 
2.51.0.618.g983fd99d29-goog