linux-kernel - [RFC PATCH 4/4] drivers/iommu-debug: Check state of mapped/unmapped kernel memory

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Message-ID: <20251003173229.1533640-5-smostafa@google.com>
Date: Fri,  3 Oct 2025 17:32:29 +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 4/4] drivers/iommu-debug: Check state of mapped/unmapped
 kernel memory

Now, as the page_ext holds count of IOMMU mappings, we can use it to
assert that any page allocated/freed is indeed not in the IOMMU.

The sanitizer doesn’t protect against mapping/unmapping during this
period. However, that’s less harmful as the page is not used by the
kernel.

Signed-off-by: Mostafa Saleh <smostafa@...gle.com>
---
 drivers/iommu/iommu-debug.c | 22 ++++++++++++++++++++++
 include/linux/iommu-debug.h |  1 +
 include/linux/mm.h          |  7 +++++++
 3 files changed, 30 insertions(+)

diff --git a/drivers/iommu/iommu-debug.c b/drivers/iommu/iommu-debug.c
index cec8f594c7fa..09157fef697e 100644
--- a/drivers/iommu/iommu-debug.c
+++ b/drivers/iommu/iommu-debug.c
@@ -71,6 +71,28 @@ static size_t iommu_debug_page_size(struct iommu_domain *domain)
 	return 1UL << __ffs(domain->pgsize_bitmap);
 }
 
+static unsigned int iommu_debug_page_count(unsigned long phys)
+{
+	unsigned int ref;
+	struct page_ext *page_ext = get_iommu_page_ext(phys);
+	struct iommu_debug_metadate *d = get_iommu_data(page_ext);
+
+	ref = atomic_read(&d->ref);
+	page_ext_put(page_ext);
+	return ref;
+}
+
+void iommu_debug_check_unmapped(const struct page *page, int numpages)
+{
+	if (!static_branch_likely(&iommu_debug_initialized))
+		return;
+
+	while (numpages--) {
+		WARN_ON(iommu_debug_page_count(page_to_phys(page)));
+		page++;
+	}
+}
+
 void iommu_debug_map(struct iommu_domain *domain, phys_addr_t phys, size_t size)
 {
 	size_t off;
diff --git a/include/linux/iommu-debug.h b/include/linux/iommu-debug.h
index 8d3ea661660f..aaf893cfafd0 100644
--- a/include/linux/iommu-debug.h
+++ b/include/linux/iommu-debug.h
@@ -17,6 +17,7 @@ void iommu_debug_map(struct iommu_domain *domain, phys_addr_t phys, size_t size)
 void iommu_debug_unmap(struct iommu_domain *domain, unsigned long iova, size_t size);
 void iommu_debug_remap(struct iommu_domain *domain, unsigned long iova, size_t size);
 void iommu_debug_init(void);
+void iommu_debug_check_unmapped(const struct page *page, int numpages);
 
 #endif /* CONFIG_IOMMU_DEBUG_PAGEALLOC */
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 06978b4dbeb8..00f5de44faa0 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -36,6 +36,7 @@
 #include <linux/rcuwait.h>
 #include <linux/bitmap.h>
 #include <linux/bitops.h>
+#include <linux/iommu-debug.h>
 
 struct mempolicy;
 struct anon_vma;
@@ -3806,12 +3807,18 @@ extern void __kernel_map_pages(struct page *page, int numpages, int enable);
 #ifdef CONFIG_DEBUG_PAGEALLOC
 static inline void debug_pagealloc_map_pages(struct page *page, int numpages)
 {
+#ifdef CONFIG_IOMMU_DEBUG_PAGEALLOC
+	iommu_debug_check_unmapped(page, numpages);
+#endif
 	if (debug_pagealloc_enabled_static())
 		__kernel_map_pages(page, numpages, 1);
 }
 
 static inline void debug_pagealloc_unmap_pages(struct page *page, int numpages)
 {
+#ifdef CONFIG_IOMMU_DEBUG_PAGEALLOC
+	iommu_debug_check_unmapped(page, numpages);
+#endif
 	if (debug_pagealloc_enabled_static())
 		__kernel_map_pages(page, numpages, 0);
 }
-- 
2.51.0.618.g983fd99d29-goog